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Microemulsions Properties And Applications 1st Edition Monzer Fanun

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Microemulsions Properties And Applications 1st Edition Monzer Fanun Microemulsions Properties And Applications 1st Edition Monzer Fanun Microemulsions Properties And Applications 1st Edition Monzer Fanun

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MICROEMULSIONS Properties and Applications 89595_C000.indd i 89595_C000.indd i 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
DANIEL BLANKSCHTEIN Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts S. KARABORNI Shell International Petroleum Company Limited London, England LISA B. QUENCER The Dow Chemical Company Midland, Michigan JOHN F. SCAMEHORN Institute for Applied Surfactant Research University of Oklahoma Norman, Oklahoma P. SOMASUNDARAN Henry Krumb School of Mines Columbia University New York, New York ERIC W. KALER Department of Chemical Engineering University of Delaware Newark, Delaware CLARENCE MILLER Department of Chemical Engineering Rice University Houston, Texas DON RUBINGH The Procter & Gamble Company Cincinnati, Ohio BEREND SMIT Shell International Oil Products B.V. Amsterdam, The Netherlands JOHN TEXTER Strider Research Corporation Rochester, New York SURFACTANT SCIENCE SERIES FOUNDING EDITOR MARTIN J. SCHICK 1918–1998 SERIES EDITOR ARTHUR T. HUBBARD Santa Barbara Science Project Santa Barbara, California ADVISORY BOARD 89595_C000.indd ii 89595_C000.indd ii 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
1. Nonionic Surfactants, edited by Martin J. Schick (see also Volumes 19, 23, and 60) 2. Solvent Properties of Surfactant Solutions, edited by Kozo Shinoda (see Volume 55) 3. Surfactant Biodegradation, R. D. Swisher (see Volume 18) 4. Cationic Surfactants, edited by Eric Jungermann (see also Volumes 34, 37, and 53) 5. Detergency: Theory and Test Methods (in three parts), edited by W. G. Cutler and R. C. Davis (see also Volume 20) 6. Emulsions and Emulsion Technology (in three parts), edited by Kenneth J. Lissant 7. Anionic Surfactants (in two parts), edited by Warner M. Linfield (see Volume 56) 8. Anionic Surfactants: Chemical Analysis, edited by John Cross 9. Stabilization of Colloidal Dispersions by Polymer Adsorption, Tatsuo Sato and Richard Ruch 10. Anionic Surfactants: Biochemistry, Toxicology, Dermatology, edited by Christian Gloxhuber (see Volume 43) 11. Anionic Surfactants: Physical Chemistry of Surfactant Action, edited by E. H. Lucassen-Reynders 12. Amphoteric Surfactants, edited by B. R. Bluestein and Clifford L. Hilton (see Volume 59) 13. Demulsification: Industrial Applications, Kenneth J. Lissant 14. Surfactants in Textile Processing, Arved Datyner 15. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, edited by Ayao Kitahara and Akira Watanabe 16. Surfactants in Cosmetics, edited by Martin M. Rieger (see Volume 68) 17. Interfacial Phenomena: Equilibrium and Dynamic Effects, Clarence A. Miller and P. Neogi 18. Surfactant Biodegradation: Second Edition, Revised and Expanded, R. D. Swisher 19. Nonionic Surfactants: Chemical Analysis, edited by John Cross 20. Detergency: Theory and Technology, edited by W. Gale Cutler and Erik Kissa 21. Interfacial Phenomena in Apolar Media, edited by Hans-Friedrich Eicke and Geoffrey D. Parfitt 22. Surfactant Solutions: New Methods of Investigation, edited by Raoul Zana 23. Nonionic Surfactants: Physical Chemistry, edited by Martin J. Schick 24. Microemulsion Systems, edited by Henri L. Rosano and Marc Clausse 25. Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C. C. Gray 26. Surfactants in Emerging Technologies, edited by Milton J. Rosen 27. Reagents in Mineral Technology, edited by P. Somasundaran and Brij M. Moudgil 28. Surfactants in Chemical/Process Engineering, edited by Darsh T. Wasan, Martin E. Ginn, and Dinesh O. Shah 29. Thin Liquid Films, edited by I. B. Ivanov 89595_C000.indd iii 89595_C000.indd iii 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
30. Microemulsions and Related Systems: Formulation, Solvency, and Physical Properties, edited by Maurice Bourrel and Robert S. Schechter 31. Crystallization and Polymorphism of Fats and Fatty Acids, edited by Nissim Garti and Kiyotaka Sato 32. Interfacial Phenomena in Coal Technology, edited by Gregory D. Botsaris and Yuli M. Glazman 33. Surfactant-Based Separation Processes, edited by John F. Scamehorn and Jeffrey H. Harwell 34. Cationic Surfactants: Organic Chemistry, edited by James M. Richmond 35. Alkylene Oxides and Their Polymers, F. E. Bailey, Jr., and Joseph V. Koleske 36. Interfacial Phenomena in Petroleum Recovery, edited by Norman R. Morrow 37. Cationic Surfactants: Physical Chemistry, edited by Donn N. Rubingh and Paul M. Holland 38. Kinetics and Catalysis in Microheterogeneous Systems, edited by M. Grätzel and K. Kalyanasundaram 39. Interfacial Phenomena in Biological Systems, edited by Max Bender 40. Analysis of Surfactants, Thomas M. Schmitt (see Volume 96) 41. Light Scattering by Liquid Surfaces and Complementary Techniques, edited by Dominique Langevin 42. Polymeric Surfactants, Irja Piirma 43. Anionic Surfactants: Biochemistry, Toxicology, Dermatology, Second Edition, Revised and Expanded, edited by Christian Gloxhuber and Klaus Künstler 44. Organized Solutions: Surfactants in Science and Technology, edited by Stig E. Friberg and Björn Lindman 45. Defoaming: Theory and Industrial Applications, edited by P. R. Garrett 46. Mixed Surfactant Systems, edited by Keizo Ogino and Masahiko Abe 47. Coagulation and Flocculation: Theory and Applications, edited by Bohuslav Dobiás 48. Biosurfactants: Production Properties Applications, edited by Naim Kosaric 49. Wettability, edited by John C. Berg 50. Fluorinated Surfactants: Synthesis Properties Applications, Erik Kissa 51. Surface and Colloid Chemistry in Advanced Ceramics Processing, edited by Robert J. Pugh and Lennart Bergström 52. Technological Applications of Dispersions, edited by Robert B. McKay 53. Cationic Surfactants: Analytical and Biological Evaluation, edited by John Cross and Edward J. Singer 54. Surfactants in Agrochemicals, Tharwat F. Tadros 55. Solubilization in Surfactant Aggregates, edited by Sherril D. Christian and John F. Scamehorn 56. Anionic Surfactants: Organic Chemistry, edited by Helmut W. Stache 57. Foams: Theory, Measurements, and Applications, edited by Robert K. Prud’homme and Saad A. Khan 58. The Preparation of Dispersions in Liquids, H. N. Stein 59. Amphoteric Surfactants: Second Edition, edited by Eric G. Lomax 89595_C000.indd iv 89595_C000.indd iv 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
60. Nonionic Surfactants: Polyoxyalkylene Block Copolymers, edited by Vaughn M. Nace 61. Emulsions and Emulsion Stability, edited by Johan Sjöblom 62. Vesicles, edited by Morton Rosoff 63. Applied Surface Thermodynamics, edited by A. W. Neumann and Jan K. Spelt 64. Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal 65. Detergents in the Environment, edited by Milan Johann Schwuger 66. Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu Kunieda 67. Liquid Detergents, edited by Kuo-Yann Lai 68. Surfactants in Cosmetics: Second Edition, Revised and Expanded, edited by Martin M. Rieger and Linda D. Rhein 69. Enzymes in Detergency, edited by Jan H. van Ee, Onno Misset, and Erik J. Baas 70. Structure-Performance Relationships in Surfactants, edited by Kunio Esumi and Minoru Ueno 71. Powdered Detergents, edited by Michael S. Showell 72. Nonionic Surfactants: Organic Chemistry, edited by Nico M. van Os 73. Anionic Surfactants: Analytical Chemistry, Second Edition, Revised and Expanded, edited by John Cross 74. Novel Surfactants: Preparation, Applications, and Biodegradability, edited by Krister Holmberg 75. Biopolymers at Interfaces, edited by Martin Malmsten 76. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, Second Edition, Revised and Expanded, edited by Hiroyuki Ohshima and Kunio Furusawa 77. Polymer-Surfactant Systems, edited by Jan C. T. Kwak 78. Surfaces of Nanoparticles and Porous Materials, edited by James A. Schwarz and Cristian I. Contescu 79. Surface Chemistry and Electrochemistry of Membranes, edited by Torben Smith Sørensen 80. Interfacial Phenomena in Chromatography, edited by Emile Pefferkorn 81. Solid–Liquid Dispersions, Bohuslav Dobiás, Xueping Qiu, and Wolfgang von Rybinski 82. Handbook of Detergents, editor in chief: Uri Zoller Part A: Properties, edited by Guy Broze 83. Modern Characterization Methods of Surfactant Systems, edited by Bernard P. Binks 84. Dispersions: Characterization, Testing, and Measurement, Erik Kissa 85. Interfacial Forces and Fields: Theory and Applications, edited by Jyh-Ping Hsu 86. Silicone Surfactants, edited by Randal M. Hill 87. Surface Characterization Methods: Principles, Techniques, and Applications, edited by Andrew J. Milling 88. Interfacial Dynamics, edited by Nikola Kallay 89. Computational Methods in Surface and Colloid Science, edited by Malgorzata Borówko 89595_C000.indd v 89595_C000.indd v 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
90. Adsorption on Silica Surfaces, edited by Eugène Papirer 91. Nonionic Surfactants: Alkyl Polyglucosides, edited by Dieter Balzer and Harald Lüders 92. Fine Particles: Synthesis, Characterization, and Mechanisms of Growth, edited by Tadao Sugimoto 93. Thermal Behavior of Dispersed Systems, edited by Nissim Garti 94. Surface Characteristics of Fibers and Textiles, edited by Christopher M. Pastore and Paul Kiekens 95. Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications, edited by Alexander G. Volkov 96. Analysis of Surfactants: Second Edition, Revised and Expanded, Thomas M. Schmitt 97. Fluorinated Surfactants and Repellents: Second Edition, Revised and Expanded, Erik Kissa 98. Detergency of Specialty Surfactants, edited by Floyd E. Friedli 99. Physical Chemistry of Polyelectrolytes, edited by Tsetska Radeva 100. Reactions and Synthesis in Surfactant Systems, edited by John Texter 101. Protein-Based Surfactants: Synthesis, Physicochemical Properties, and Applications, edited by Ifendu A. Nnanna and Jiding Xia 102. Chemical Properties of Material Surfaces, Marek Kosmulski 103. Oxide Surfaces, edited by James A. Wingrave 104. Polymers in Particulate Systems: Properties and Applications, edited by Vincent A. Hackley, P. Somasundaran, and Jennifer A. Lewis 105. Colloid and Surface Properties of Clays and Related Minerals, Rossman F. Giese and Carel J. van Oss 106. Interfacial Electrokinetics and Electrophoresis, edited by Ángel V. Delgado 107. Adsorption: Theory, Modeling, and Analysis, edited by József Tóth 108. Interfacial Applications in Environmental Engineering, edited by Mark A. Keane 109. Adsorption and Aggregation of Surfactants in Solution, edited by K. L. Mittal and Dinesh O. Shah 110. Biopolymers at Interfaces: Second Edition, Revised and Expanded, edited by Martin Malmsten 111. Biomolecular Films: Design, Function, and Applications, edited by James F. Rusling 112. Structure–Performance Relationships in Surfactants: Second Edition, Revised and Expanded, edited by Kunio Esumi and Minoru Ueno 113. Liquid Interfacial Systems: Oscillations and Instability, Rudolph V. Birikh, Vladimir A. Briskman, Manuel G. Velarde, and Jean-Claude Legros 114. Novel Surfactants: Preparation, Applications, and Biodegradability: Second Edition, Revised and Expanded, edited by Krister Holmberg 115. Colloidal Polymers: Synthesis and Characterization, edited by Abdelhamid Elaissari 116. Colloidal Biomolecules, Biomaterials, and Biomedical Applications, edited by Abdelhamid Elaissari 117. Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications, edited by Raoul Zana and Jiding Xia 89595_C000.indd vi 89595_C000.indd vi 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
118. Colloidal Science of Flotation, Anh V. Nguyen and Hans Joachim Schulze 119. Surface and Interfacial Tension: Measurement, Theory, and Applications, edited by Stanley Hartland 120. Microporous Media: Synthesis, Properties, and Modeling, Freddy Romm 121. Handbook of Detergents, editor in chief: Uri Zoller, Part B: Environmental Impact, edited by Uri Zoller 122. Luminous Chemical Vapor Deposition and Interface Engineering, HirotsuguYasuda 123. Handbook of Detergents, editor in chief: Uri Zoller, Part C: Analysis, edited by Heinrich Waldhoff and Rüdiger Spilker 124. Mixed Surfactant Systems: Second Edition, Revised and Expanded, edited by Masahiko Abe and John F. Scamehorn 125. Dynamics of Surfactant Self-Assemblies: Micelles, Microemulsions, Vesicles and Lyotropic Phases, edited by Raoul Zana 126. Coagulation and Flocculation: Second Edition, edited by Hansjoachim Stechemesser and Bohulav Dobiás 127. Bicontinuous Liquid Crystals, edited by Matthew L. Lynch and Patrick T. Spicer 128. Handbook of Detergents, editor in chief: Uri Zoller, Part D: Formulation, edited by Michael S. Showell 129. Liquid Detergents: Second Edition, edited by Kuo-Yann Lai 130. Finely Dispersed Particles: Micro-, Nano-, and Atto-Engineering, edited by Aleksandar M. Spasic and Jyh-Ping Hsu 131. Colloidal Silica: Fundamentals and Applications, edited by Horacio E. Bergna and William O. Roberts 132. Emulsions and Emulsion Stability, Second Edition, edited by Johan Sjöblom 133. Micellar Catalysis, Mohammad Niyaz Khan 134. Molecular and Colloidal Electro-Optics, Stoyl P. Stoylov and Maria V. Stoimenova 135. Surfactants in Personal Care Products and Decorative Cosmetics, Third Edition, edited by Linda D. Rhein, Mitchell Schlossman, Anthony O'Lenick, and P. Somasundaran 136. Rheology of Particulate Dispersions and Composites, Rajinder Pal 137. Powders and Fibers: Interfacial Science and Applications, edited by Michel Nardin and Eugène Papirer 138. Wetting and Spreading Dynamics, edited by Victor Starov, Manuel G. Velarde, and Clayton Radke 139. Interfacial Phenomena: Equilibrium and Dynamic Effects, Second Edition, edited by Clarence A. Miller and P. Neogi 140. Giant Micelles: Properties and Applications, edited by Raoul Zana and Eric W. Kaler 141. Handbook of Detergents, editor in chief: Uri Zoller, Part E: Applications, edited by Uri Zoller 142. Handbook of Detergents, editor in chief: Uri Zoller, Part F: Production, edited by Uri Zoller and co-edited by Paul Sosis 143. Sugar-Based Surfactants: Fundamentals and Applications, edited by Cristóbal Carnero Ruiz 144. Microemulsions: Properties and Applications, edited by Monzer Fanun 89595_C000.indd vii 89595_C000.indd vii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
89595_C000.indd viii 89595_C000.indd viii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
MICROEMULSIONS Edited by Monzer Fanun Al-Quds University East Jerusalem, Palestine CRC Press is an imprint of the Taylor & Francis Group, an informa business Boca Raton London New York Properties and Applications 89595_C000.indd ix 89595_C000.indd ix 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-8959-2 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, trans- mitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright. com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Fanun, Monzer. Microemulsions : properties and applications / Monzer Fanun. p. cm. -- (Surfactant science ; 144) ISBN 978-1-4200-8959-2 (alk. paper) 1. Emulsions. I. Title. TP156.E6F36 2008 660’.294514--dc22 2008029538 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com 89595_C000.indd x 89595_C000.indd x 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
Contents Foreword ............................................................................................................. xv Preface................................................................................................................xxi Editor ................................................................................................................xxv Contributors ....................................................................................................xxvii Chapter 1 A Phase Diagram Approach to Microemulsions.............................1 Stig E. Friberg and Patricia A. Aikens Chapter 2 Physicochemistry of W/O Microemulsions: Formation, Stability, and Droplet Clustering................................................... 17 Animesh Kumar Rakshit and Satya Priya Moulik Chapter 3 Percolating Phenomenon in Microemulsions: Effect of External Entity ............................................................... 59 S. K. Mehta, Khushwinder Kaur, Gurpreet Kaur, and K. K. Bhasin Chapter 4 Influence of Polyethylene Glycols and Polyethylene Glycol Dimethyl Ethers upon the Internal Dynamics of Water in Oil Microemulsions................................................................... 77 A. Cid, L. García-Río, D. Gómez-Díaz, and J. C. Mejuto Chapter 5 Microemulsions with Mixed Nonionic Surfactants ...................... 87 Monzer Fanun Chapter 6 Simple Alcohols and Their Role in the Structure and Interactions of Microemulsion Systems ................................143 Matija Tomšič and Andrej Jamnik Chapter 7 Formation and Characterization of Emulsified Microemulsions........................................................................... 185 Anan Yaghmur, Liliana de Campo, and Otto Glatter xi 89595_C000.indd xi 89595_C000.indd xi 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
xii Contents Chapter 8 Dynamics of Solvent and Rotational Relaxation of RTILs in RTILs-Containing Microemulsions........................................ 203 Debabrata Seth and Nilmoni Sarkar Chapter 9 Microemulsion Systems and Their Potential as Drug Carriers.......................................................................... 247 Raid G. Alany, Gamal M. M. El Maghraby, Karen Krauel-Goellner, and Anja Graf Chapter 10 Physicochemical Characterization of Pharmaceutically Applicable Microemulsions: Tween 40/Imwitor 308/ Isopropyl Myristate/Water .......................................................... 293 Mirjana Gašperlin and Marija Bešter-Rogač Chapter 11 Places of Microemulsion and Emulsion in Cancer Therapy: In Vitro and In Vivo Evaluation...................................................313 Ercüment Karasulu, Burçak Karaca, Levent Alparslan, and H. Yesim Karasulu Chapter 12 Enzyme Kinetics as a Useful Probe for Micelle and Microemulsion Structure and Dynamics ..............................331 Werner Kunz, Didier Touraud, and Pierre Bauduin Chapter 13 Biocatalysis in Microemulsions .................................................. 349 A. Xenakis, V. Papadimitriou, H. Stamatis, and F. N. Kolisis Chapter 14 Microemulsions as Decontamination Media for Chemical Weapons and Toxic Industrial Chemicals................................... 387 Thomas Hellweg, Stefan Wellert, Hans-Juergen Altmann, and André Richardt Chapter 15 Microemulsions as Potential Interfacial Chemical Systems Applied in the Petroleum Industry...............................................411 Afonso Avelino Dantas Neto, Tereza Neuma de Castro Dantas, Maria Carlenise Paiva de Alencar Moura, Eduardo Lins de Barros Neto, and Alexandre Gurgel 89595_C000.indd xii 89595_C000.indd xii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
Contents xiii Chapter 16 Nanoparticle Formation in Microemulsions: Mechanism and Monte Carlo Simulations...................................................... 451 M. de Dios, F. Barroso, and C. Tojo Chapter 17 Nanoparticle Uptake by (W/O) Microemulsions.........................465 Maen M. Husein and Nashaat N. Nassar Chapter 18 TiO2 Nanoparticles in Microemulsion: Photophysical Properties and Interfacial Electron Transfer Dynamics.............. 483 Hirendra N. Ghosh Chapter 19 Microemulsions as Pseudostationary Phases in Electrokinetic Chromatography: I. Estimation of Physicochemical Parameters. II. Analysis of Drugs in Pharmaceutical and Biofluidic Matrices ............................................................... 501 Valeria Tripodi and Silvia Lucangioli Index................................................................................................................ 527 89595_C000.indd xiii 89595_C000.indd xiii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
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Foreword MICROEMULSIONS—A VITAL FUNDAMENTAL RESEARCH AREA MOVING RAPIDLY INTO APPLICATIONS WHILE HAVING ITS SCIENTIFIC BASIS IN OTHER SURFACTANT SELF-ASSEMBLY SYSTEMS This book focuses on the properties and applications of microemulsions and, in particular, on their interrelationship. Of late, microemulsions have become a pop- ular subject and applications are emerging rapidly; this further stimulates the fun- damental studies. Therefore, this book is very timely and I congratulate the editor, Professor Monzer Fanun, for having prepared a volume with this focus and, in particular, achieving this so well by assembling an impressive list of contributors; this list is a good mix of established leading scientists and young colleagues enter- ing the field recently as they will be the ones who will continue to develop our research area. The history of microemulsions has been full of ups and downs and has been involved in many heated controversies. The name “microemulsions” itself has no doubt contributed strongly to confusion. Microemulsions are not micro but nano and are not emulsions. The history of microemulsion research is complex [1] and needs to be recounted since it provides important lessons. Here I rather wish to make a few comments from my experience with the evolution of microemulsion, which have a direct bearing on this book and its relevance. What is a microemulsion? This question was very much in focus when I first came in contact with the field by the end of the 1960s and early 1970s. The very fact that a question like this arises leads to considerable confusion and unneces- sary work. Thus, had the true nature of microemulsions been understood, a resort to the basic literature on surfactant self-assembly would have given logical expla- nations to many observations. Thirty years ago, when I had just been appointed to the chair of physical chemistry at Lund University, Professor Ingvar Danielsson from Åbo Akademi in Turku, Finland came for a sabbatical. Åbo Akademi was the world-famous institu- tion for physical chemistry where the founder of the institution, Per Ekwall, along with pupils such as Ingvar Danielsson, Krister Fontell, and Leo Mandell, had developed much of our fundamental understanding of surfactant systems, including micellization, phase behavior, and liquid crystallinity. On his retirement from Åbo, Ekwall moved to Stockholm to found the Laboratory (later Institute) of Surface Chemistry, while Danielsson took over his chair in Åbo. My first contacts with surfactant science and much of my learning were with this Stockholm-Åbo research community. xv 89595_C000.indd xv 89595_C000.indd xv 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
Having been concerned with aspects of surfactant aggregation on the macro- scopic and aggregate levels, Danielsson took interest in a deeper molecular level of understanding, using some novel nuclear magnetic resonance (NMR) approaches, which I had developed along with my colleagues in Lund. These studies included microemulsions and, discussing the research results and reading the literature, we became more and more concerned about the fact that different authors had very different opinions on microemulsions. (It is interesting that Ekwall and Fontell refused to use this term even though they were behind some of the pioneering and still central observations on microemulsions. Since the term referred to thermodynamically stable solutions, they found it a misno- mer.) Therefore, we found it timely to suggest a definition of microemulsion as the following: A microemulsion is a system of water, oil, and amphiphile, which is a single optically isotropic and thermodynamically stable liquid solution [2]. We also gave several examples of what we considered should be included in microemulsions and what should not. Looking into the contents of this book, and contemplating Monzer’s invita- tion to write this foreword, I found it of interest to examine a little the accep- tance of our definition among colleagues. While having a general impression that, after a quite long period of questioning, it became more and more accepted, I found it of interest to examine this further by a citation analysis. Our short note is certainly not a significant scientific contribution, but it is quite well cited (and is in fact among my 10 most cited papers). However, the citations show a very unusual variation over time, the distribution being pronouncedly “bimodal.” In the first years after publication, there is quite a constant modest citation frequency. Thereafter, there is a very pronounced peak in 1989, indicating that this is the year that a more general acceptance was obtained. Afterwards, citations decrease strongly and one would have expected that the paper would as usual start to become forgotten. However, a few years ago, citations started to increase in number again and, from the citations during the first half of 2008, we can guess that this year will give the largest number of citations so far. Why is that so? Some clue can be obtained from the field of the journals where the paper is cited. Thus in 1989, most citations were in journals that focused more on physical and colloid chemistry. The pattern is very different in 2008. A majority of the citations are in journals dealing with more applied aspects, in particular, in the pharmaceutical sciences. Is there any other evidence that microemulsions are now becoming better understood in the applied sciences, like pharmaceutics? An indication can be obtained by considering textbooks. A leading textbook in pharmacy is Physico- chemical Principles of Pharmacy by A. T. Florence and D. Attwood [3]. Both the placing of microemulsions in the book and the text devoted to this topic reveal that even in 1998, when the third edition was published, microemulsions had received very little attention in the pharmaceutical field and that, furthermore, the nature of microemulsions was misunderstood. Thus, while there were lengthy multipage descriptions of surfactant micellization, liquid crystallinity, vesicles, and solubi- lization, microemulsions were dealt with in a mere seven line paragraph starting xvi Foreword 89595_C000.indd xvi 89595_C000.indd xvi 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
Foreword xvii “Microemulsions, or so-called swollen micellar systems, consist of apparently homogeneous transparent systems of low viscosity which contain a high percent- age of both oil and water and high concentrations (15%–25%) of emulsifier mixture.” The misconception of microemulsions in the pharmaceutical field is accentuated by the fact that rather than being placed together with other thermo- dynamically stable surfactant self-assembly systems, it is considered as a type of dispersion and placed under the general heading “Emulsions, Suspensions, and Other Dispersions.” It is indicated from the citation analysis mentioned that if a corresponding textbook is prepared today, microemulsions would receive much more attention and would be properly classified and treated in conjunction with related surfactant systems, like micelles and liquid crystals, as they have indeed been in textbooks of physical chemistry and colloid chemistry for a long time. This book contains significant contributions regarding the applications of microemulsions for pharmaceutical formulations, as well as for other applications, and will no doubt help considerably to provide an excellent basis for applications into new fields. Regarding the long-standing issue of the confusion of treating microemulsions as one type of emulsion, Chapter 7 by Otto Glatter and coauthors, dealing with emulsified microemulsions, is particularly enlightening as it clearly hints to this misconception. Stig Friberg was certainly the pioneer who demonstrated that microemulsions are indeed thermodynamically stable solutions and, therefore, should be described by phase diagrams with respect to their stability. The significance of his work on the phase behavior of surfactant–oil–water systems for the development of the microemulsion field cannot be overestimated and it is indeed very appropriate that he was invited to write the first chapter of this book. I was myself very fortunate to have early contacts with Stig Friberg. In addition, I was strongly influenced and helped by the phase diagram work of two other pioneers in the field, Per Ekwall, already mentioned above, and Kozo Shinoda in Yokohama. Several of my collaborations with Friberg, Shinoda, and Ekwall concerned microemulsion microstructure, where they provided enlightening systems for structural investigation and deep insight into the subject. I consider my most important contribution to the field of microemulsion as being the first, together with coworkers, to demonstrate microemulsion bicontinu- ity. However, this work also nicely demonstrates how important it is in microemul- sionresearchtohaveabroaderperspective,inparticular considering other surfactant phases. My first study dealing with surfactant phase bicontinuity did not thus concern microemulsions but cubic liquid crystalline phases. In preparing a chapter dealing with applications of NMR for a book on Liquid Crystals and Plastic Crystals [4], I became confused when I came to the cubic phases.As we know, cubic phases can be located in different concentration ranges in a phase diagram, inter alia between the micellar solutions and the normal hexagonal phase, and between the hexagonal and the lamellar phases. I soon realized that the surfactant self-diffusion would be very different for discrete aggregates and for connected structures. This would 89595_C000.indd xvii 89595_C000.indd xvii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
thus be an interesting possibility for solving the problem of the structure of cubic liquid crystalline phases. A few experiments with a postdoctoral fellow, Tom Bull, at the new pulsed NMR spectrometer, giving differences in surfactant diffusion by orders of magnitude between the two cubic phases, could directly prove that one was built up of discrete micelles while the other was bicontinuous [5]. The cubic phase, which is more dilute in surfactant, was thus found to be characterized by very slow surfactant diffusion and thus must consist of (more or less stationary) discrete aggregates. In the more concentrated cubic phase, surfactant diffusion was found to be more than one order of magnitude faster. This rather surprising finding could only be understood if the surfactant molecules could diffuse freely over macroscopic distances; thus surfactant aggregates are connected. The distinction between discrete “droplet” structures and bicontinuous ones became central in the subsequent studies on microemulsions in Lund [6–12]. This research topic became even more emphasized when Peter Stilbs introduced the Fourier transform version of the NMR technique [13–16]. That surfactant self-assembly systems, which include liquid crystalline phases and isotropic solutions, can be divided into those that have discrete self-assembly aggregates and those where the aggregates are connected in one, two, or three dimensions was very clear for the pioneers of the microemulsion field mentioned above. Regarding lamellar phases, the two-dimensional connectivity was already appreciated at a very early stage. The same holds true for the (“normal” and “reverse”) hexagonal phases, although erroneous models of linearly associated spherical micelles, “pearls-on-a-string,” can be found in the literature; such a linear association was also, again incorrectly, advanced to explain droplet growth in microemulsions. The general acceptance of connectivity for these anisotropic phases stood in sharp contrast to a great difficulty to get an acceptance for bicontinu- ity for other phases. This is partly related to the fact that contrary to these anisotro- pic phases, it has been much more difficult to structurally characterize the different isotropic phases found in simple and complex surfactant systems: cubic liquid crystals, solutions in binary surfactant–water systems, and microemulsions. The first verification was due to observations of molecular self-diffusion over macro- scopic distances. Electrical conductivity offers a partial insight in providing infor- mation on the extension of aqueous domains. Fluorescence quenching can provide information on the growth of nonpolar domains, but a probe has to be introduced. Later cryogenic transmission electron microscopy has developed into a very impor- tant tool for imaging different surfactant phases. Using a similar approach as for cubic phases, it was thus quite straightforward to address the problem of microemulsion structure. Thus, by measuring oil and water self-diffusion, it was quite easy to establish whether oil or water or none of them are confined to discrete domains, “droplets.” In the first work on microemul- sion structure by self-diffusion, using both tracer techniques and NMR spin-echo measurements, it was clearly shown that, in addition to droplet microemulsions, over wide ranges of composition they can be bicontinuous [6]; this is manifested by both oil and water diffusion being rapid, not much less than the self-diffusion of the neat liquids. xviii Foreword 89595_C000.indd xviii 89595_C000.indd xviii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
Foreword xix The self-diffusion approach to microstructure is not limited to cubic phases or microemulsions. An early study concerned the demonstration of micellar growth into worm-like structures for nonionic surfactants [17,18]. Parallel pioneering studies on phase behavior of nonionic surfactants by Gordon Tiddy [19] also illus- trated the same feature. Another problem, soon to be tackled, was that of the microstructure of the “sponge phases,” a “microemulsion analogue,” for binary surfactant systems, termed L3 by Ekwall (and identified by him in a number of systems). While isotropic solutions in simple surfactant–water mixtures were for a long time considered synonymous with solutions of discrete surfactant micelles, there were indications of a more complex situation given by the clouding and phase separation into two solutions of nonionic surfactants at elevated temperature. Here self-diffusion was again expected to provide the solution [19,20]. For the sponge phase, water diffusion was much reduced compared to classical micellar solu- tions. In fact, it was close to 2/3 of the value of neat water. The surfactant diffusion was, on the other hand, found to be much more rapid, and close to 2/3 of the dif- fusion of the neat liquid surfactant, than what was observed for previously studied micellar solutions. The solutions are thus bicontinuous. These systems are perfect illustrations of bicontinuity and in many respects useful models of bicontinuous microemulsions. Both the water and surfactant self-diffusion coefficients are close to 2/3 of the values of the neat liquids, corresponding to an ideal zero mean curva- ture bicontinuous structure. While I have illustrated here, with some examples from our own research, how progress in our understanding has been dependent on understanding alterna- tive surfactant phases, this approach is certainly not unique. Several pioneers like Friberg, Ekwall, Shinoda, Tiddy, Scriven, and Wennerström, have provided beau- tiful examples of such a “holistic” view. It is my firm belief that in the ongoing expan- sion of the microemulsion field, that the present book emphasizes and supports a broader look into surfactant self-assembly and a resort to simpler surfactant sys- tems are mandatory. Björn Lindman Coimbra University and Lund University REFERENCES 1. Lindman, B.; Friberg, S. Microemulsions—a historical overview. In Handbook of Microemulsion Science and Technology, P. Kumar and K. L. Mittal, eds. Marcel Dekker, New York, 1999, pp. 1–12. 2. Danielsson, I.; Lindman, B. The definition of microemulsion. Colloids Surfaces 3, 1981, 391–392. 3. Florence, A.T; Attwood, D. Physicochemical Principles of Pharmacy, 3rd edn, Pharmaceutical Press, London, 1998. 4. Johansson, Å.; Lindman, B. In Liquid Crystals and Plastic Crystals, Nuclear Magnetic Resonance Spectroscopy of Liquid Crystals-Amphiphilic Systems, G.W. Gray and P. A. Winsor, eds. Ellis Horwood Publishers, Chichester, 1974, Vol. 2, pp. 192–230. 89595_C000.indd xix 89595_C000.indd xix 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
5. Bull, T.; Lindman, B. Amphiphile diffusion in cubic lyotropic mesophases. Mol. Cryst. Liquid Cryst. 28, 1975, 155–160. 6. Lindman, B.; Kamenka, N.; Kathopoulis, T.M.; Brun, B.; Nilsson, P.G. Translational diffusion and solution structure of microemulsions. J. Phys. Chem. 84, 1980, 2485–2490. 7. Nilsson, P. G.; Lindman, B. Solution structure of nonionic surfactant microemul- sions from NMR self-diffusion studies. J. Phys. Chem. 86, 1982, 271–279. 8. Guéring, P.; Lindman, B. Droplet and bicontinuous structures in cosurfactant micro- emulsions from multi-component self-diffusion measurements. Langmuir 1, 1985, 464–468. 9. Olsson, U.; Shinoda, K.; Lindman, B. Change of the structure of microemulsions with the HLB of nonionic surfactant as revealed by NMR self-diffusion studies. J. Phys. Chem. 90, 1986, 4083–4088. 10. Lindman, B.; Shinoda, K.; Olsson, U.; Anderson, D.; Karlström, G.; Wennerström, H. On the demonstration of bicontinuous structures in microemulsions. Colloids Surfaces 38, 1989, 205–224. 11. Lindman, B.; Olsson, U. Structure of microemulsions studied by NMR Ber. Bunsen- ges. Phys. Chem. 100, 1996, 344–363. 12. Shinoda, K.; Lindman, B. Organized surfactant systems: Microemulsions. Lang- muir 3, 1987, 135–149. 13. Stilbs, P.; Moseley, M. E. Nuclear spin-echo experiments on standard Fourier-trans- form NMR spectrometers—Application to multi-component self-diffusion studies. Chem. Scripta 13, 1979, 26–28. 14. Stilbs, P. Prog. Fourier transform pulsed-gradient spin-echo studies of molecular diffusion. NMR Spectrosc. 19, 1987, 1–45. 15. Stilbs, P.; Moseley, M. E.; Lindman, B. Fourier transform NMR self-diffusion mea- surements on microemulsions. J. Magn. Reson. 40, 1980, 401–404. 16. Lindman, B.; Stilbs, P.; Moseley, M. E. Fourier transform NMR self-diffusion and microemulsion structure. J. Colloid Interface Sci. 83, 1981, 569–582. 17. Nilsson, P. G.; Wennerström, H.; Lindman, B. Structure of micellar solutions of nonionic surfactants. NMR self-diffusion and proton relaxation studies of poly(ethyleneoxide) alkylethers. J. Phys. Chem. 87, 1983, 1377–1385. 18. Lindman, B.; Wennerström, H. Nonionic micelles grow with increasing tempera- ture. J. Phys. Chem. 95, 1991, 6053–6054. 19. Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Phase behaviour of polyoxyethylene surfactants with water. Mesophase structures and partial miscibility (cloud points). Chem. Soc. Faraday Trans. 79, 1983, 975–1000. 20. Nilsson, P. G.; Lindman, B. Nuclear magnetic resonance self-diffusion and proton relaxation studies of nonionic surfactant solutions. Aggregate shape in isotropic solutions above the clouding temperature. J. Phys. Chem. 88, 1984, 4764–4769. 21. Lindman, B.; Olsson, U.; Stilbs, P.; Wennerström, H. Comment on the self-diffusion in L3 and other bicontinuous surfactant solutions. Langmuir 9, 1993, 625–626. xx Foreword 89595_C000.indd xx 89595_C000.indd xx 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
xxi Preface Microemulsions are microheterogeneous, thermodynamically stable, sponta- neously formed mixtures of oil and water under certain conditions by means of surfactants, with or without the aid of a cosurfactant. The first paper on microemulsions appeared in 1943 by Hoar et al., but it was Schulman and coworkers who first proposed the word “microemulsion” in 1959. Since then, the term “microemulsions” has been used to describe multicomponent systems comprising nonpolar, aqueous, surfactant, and cosurfactant components. The application areas of microemulsions have increased dramatically during the past decades. For example, the major industrial areas are fabricating nanopar- ticles, oil recovery, pollution control, and food and pharmaceutical industries. This book is a comprehensive reference that provides a complete and system- atic assessment of all topics affecting microemulsion performance, discussing the fundamental characteristics, theories, and applications of these dispersions that have been developed over the last decade. The book opens with a chapter that describes a phase diagram approach to microemulsions by two leading authorities (Friberg and Aikens) who have con- tributed significantly to the field of microemulsions. In the next three chapters, Moulik and Rakshit, Mehta and coworkers, and Mejuto and coworkers, respectively, advance different approaches to describe the percolation phenomenon in microe- mulsion systems. Theories that predict droplet clustering along with the basic conditions required for the formation and stability of these reverse micellar systems and the composition, temperature, and pressure-dependent conductance percolation and energetics of droplet clustering are reviewed. The influence of different additives on the conductance percolation of ionic microemulsions is also reviewed. Significant progress has been made in the formulation and characterization of new microemulsion systems. Properties of microemulsions with mixed non- ionic surfactants and different types of oils are reviewed in Chapter 5 by Fanun. A comprehensive review on the influence of various simple alcohols on the internal structural organization of microemulsion systems is presented in Chapter 6 by Tomšič and Jamnik. Chapter 7 by Glatter and coworkers focuses on the effect of variations in temperature and solubilizing oil on the formation and the reversible structural transitions of emulsified microemulsions that have excellent potential in applications such as nanoreactors or host systems for solubilizing active molecules in cosmetic, pharmaceutical, and food industries. The interaction of water with room temperature ionic liquids (RTILs) has been studied in RTIL/surfactant/water-containing ternary microemulsions by solvent and rotational relaxation of neutral Coumarin probes, namely Coumarin 153 and Coumarin 151, using steady-state and picosecond time-resolved emission spectroscopy, reviewed by Seth and Sarkar in Chapter 8. 89595_C000.indd xxi 89595_C000.indd xxi 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
xxii Preface Microemulsions accommodate poorly soluble drugs (both hydrophilic and lipophilic) and protect those that are vulnerable to chemical and enzymatic deg- radation. They have the potential to increase the solubility of poorly soluble drugs, enhance the bioavailability of drugs with poor permeability, reduce patient variability, and offer an alternative for controlled drug release. In Chap- ter 9, Alany and coworkers review the formulation and characterization of microemulsions intended for drug delivery applications. Recent investigations on pharmaceutically applicable microemulsions are described in Chapter 10 by Gašperlin and Bešter-Rogač. The use of emulsions and microemulsions as a delivery system for cancer therapy is described in Chapter 11 by Karasulu and coworkers. Enzymes when hosted in reverse micelles can catalyze reactions that are not favored in aqueous media. Products of high-added value can be thus pro- duced in these media. The potential technical and commercial applications of enzyme-containing microemulsions as microreactors are mainly linked to their unique physicochemical properties. The potential biotechnological applications of microemulsions with immobilized biocatalysts such as enzymes are described in Chapter 12 by Kunz and coworkers and in Chapter 13 by Xenakis and coworkers. Great efforts have been made in order to replace established but harmful, corrosive, and therefore, obsolete decontamination media for chemical warfare agents and toxic industrial chemicals. Chapter 14 by Hellweg and coworkers discusses the considerable advantages of microemulsion-based decontamination systems with respect to practical boundary conditions and fundamental princi- ples of microemulsion formation. Additionally, the authors illustrate the further development to versatile, environmentally compatible and nonharmful systems containing nanoparticles and enzymes as active components. Several segments of the petroleum industry can be optimized with the use of microemulsions. Research has been carried out on potential microemulsified for- mulations for compression-ignition, cycle-diesel engines, which, in spite of bring- ing about a slight increase in consumption, produce less polluting emissions. In Chapter 15, Dantas Neto and coworkers summarize recent advances in microe- mulsions in this type of industry. Microemulsions can be considered as true nanoreactors, which can be used to synthesize nanomaterials. The main idea behind this technique is that by appropriate control of the synthesis parameters one can use these nanoreactors to produce tailor-made products down to a nanoscale level. Chapter 16 by Tojo and coworkers describes the use of Monte Carlo simulations to study the influence of the critical nucleus size and the chemical reaction rate on the formation of nanoparticles in micro- emulsions. Chapter 17 by Husein and Nassar focuses on exploring ways of maximizing the concentration of stable colloidal nanoparticles, nanoparticle uptake, in single (w/o) microemulsions. Chapter 18 by Ghosh describes the photophysical and interfacial electron transfer behavior of anatase TiO2 nanoparticles in microemulsions. Capillary electrophoresis is a powerful technique with relevant features of performance such as simplicity, versatility, very high resolution in short time 89595_C000.indd xxii 89595_C000.indd xxii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
Preface xxiii of analysis, and low cost of operation. The final chapter by Tripodi and Lucangioli describes the use of microemulsions in capillary electrophoresis as pseudostationary phases in the electrokinetic chromatography mode. This method has extensive applications in different fields of pharmaceutical analysis for the determination of drugs and their impurities in bulk material and pharmaceutical formulations for the dosage of drugs in biological fluids. In quintessence, this book represents the collective knowledge of young and renowned researchers and engineers in the field of microemulsions. This book covers recent advances in the characterization of the properties of microemulsions; it covers new types of materials used for the formulation and stabilization of microemulsions, and it also covers new applications. An important feature of this book is that the author of each chapter has been given the freedom to present, as he/she sees fit, the spectrum of the relevant science, from pure to applied, in his/her particular topic. Of course this approach inevitably leads to some overlap and repetition in different chapters, but that does not necessarily matter. Any author has his/her own views on, and approach to, a specific topic, molded by his/her own experience. I hope that this book will familiarize the reader with the scientific and engineering aspects of microemulsions, and provides experienced researchers, scientists, and engineers in academic and industry communities with the latest developments in this field. I would like to thank all those who contributed as chapter authors despite their busy schedules. In total, 52 individuals from 15 countries contributed to the work. All of them are recognized and respected experts in the areas they wrote about. None of them is associated with any errors or omissions that remain. I take full responsibility. Special thanks are due to the reviewers for their valuable comments as peer review is a requirement to preserve the highest standard of publication. My appreciation goes to Barbara Glunn of Taylor & Francis for her genuine interest in this project. Monzer Fanun Associate Professor Al-Quds University East Jerusalem, Palestine 89595_C000.indd xxiii 89595_C000.indd xxiii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
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xxv Editor Monzer Fanun is a professor in surface and colloid science, the head of the Colloids and Surfaces Research Laboratory, and a member of the Nanotechnology Research Group atAl-Quds University, East Jerusalem, Palestine. He has authored and coauthored more than 40 professional papers. He is a member of the European Colloid and Interface Society and a fellow of the Palestinian Academy for Science and Technology. In 2003, he received his PhD in applied chemistry from the Casali Institute of Applied Chemistry a part of the Institute of Chemistry at the Hebrew University of Jerusalem, Israel. 89595_C000.indd xxv 89595_C000.indd xxv 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
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xxvii Contributors Patricia A. Aikens BASF Corp. Stony Brook, New York Raid G. Alany School of Pharmacy University of Auckland Auckland, New Zealand Maria Carlenise Paiva de Alencar Moura Universidade Federal do Rio Grande do Norte Centro de Tecnologia Departamento de Engenharia Química UFRN—Federal University of Rio Grande do Norte Chemical Engineering Department Campus Universitário Natal, Brazil Levent Alparslan Department of Biopharmaceutics and Pharmacokinetics Faculty of Pharmacy University of Ege Izmir, Turkey and Center for Drug R&D and Pharmacokinetic Applications University of Ege Izmir, Turkey Hans-Juergen Altmann Armed Forces Scientific Institute for NBC Protection Munster, Germany Eduardo Lins de Barros Neto Universidade Federal do Rio Grande do Norte Centro de Tecnologia Departamento de Engenharia Química UFRN—Federal University of Rio Grande do Norte Chemical Engineering Department Campus Universitário Natal, Brazil F. Barroso Department of Physical Chemistry Faculty of Chemistry University of Vigo Vigo, Spain Pierre Bauduin Institut de Chimie Séparative de Marcoule Bagnols-sur-Cèze, France Marija Bešter-Rogač Faculty of Chemistry and Chemical Technology University of Ljubljana Ljubljana, Slovenia K. K. Bhasin Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India Liliana de Campo Department of Applied Mathematics The Australian National University Canberra, New South Wales, Australia 89595_C000.indd xxvii 89595_C000.indd xxvii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
xxviii Contributors Tereza Neuma de Castro Dantas Universidade Federal do Rio Grande do Norte Centro de Ciências Exatas e da Terra Departamento de Química UFRN—Federal University of Rio Grande do Norte Chemistry Department Campus Universitário Natal, Brazil A. Cid Department of Physical Chemistry Faculty of Sciences University of Vigo at Ourense Ourense, Spain Afonso Avelino Dantas Neto Universidade Federal do Rio Grande do Norte Centro de Tecnologia Departamento de Engenharia Química UFRN—Federal University of Rio Grande do Norte Chemical Engineering Department Campus Universitário Natal, Brazil M. de Dios Department of Physical Chemistry Faculty of Chemistry University of Vigo Vigo, Spain Gamal M. M. El Maghraby Department of Pharmaceutics King Saud University Riyadh, Saudi Arabia Monzer Fanun Faculty of Science and Technology Al-Quds University East Jerusalem, Palestine Stig E. Friberg Chemistry Department University of Virginia Charlottesville, Virginia L. García-Río Departamento de Química-Física Facultad de Química Universidad de Santiago de Compostela Santiago de Compostela, Spain Mirjana Gašperlin Faculty of Pharmacy University of Ljubljana Ljubljana, Slovenia Hirendra N. Ghosh Radiation and Photochemistry Division Bhabha Atomic Research Center Mumbai, Maharashtra, India Otto Glatter Institute of Chemistry University of Graz Graz, Austria D. Gómez-Díaz Departamento de Ingeniería Química Escuela Técnica Superior Universidad de Santiago de Compostela Santiago de Compostela, Spain Anja Graf School of Pharmacy University of Otago Dunedin, New Zealand 89595_C000.indd xxviii 89595_C000.indd xxviii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
Contributors xxix Alexandre Gurgel Universidade Federal de Viçosa Centro de Ciências Exatas e Tecnológicas Departamento de Química UFV—Federal University of Viçosa Chemistry Department Campus Universitário Viçosa, Brazil Thomas Hellweg Physical Chemistry I University of Bayreuth Bayreuth, Germany Maen M. Husein Department of Chemical and Petroleum Engineering University of Calgary Calgary, Alberta, Canada Andrej Jamnik Faculty of Chemistry and Chemical Technology University of Ljubljana Ljubljana, Slovenia Burçak Karaca Department of Medical Oncology School of Medicine University of Ege Izmir, Turkey Ercüment Karasulu Department of Biopharmaceutics and Pharmacokinetics Faculty of Pharmacy University of Ege Izmir, Turkey and Center for Drug R&D and Pharmacokinetic Applications University of Ege Izmir, Turkey H. Yesim Karasulu Department of Pharmaceutical Technology Faculty of Pharmacy University of Ege Izmir, Turkey Gurpreet Kaur Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India Khushwinder Kaur Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India F. N. Kolisis Institute of Biological Research and Biotechnology National Hellenic Research Foundation Athens, Greece and School of Chemical Engineering National Technical University of Athens Athens, Greece Karen Krauel-Goellner Institute of Food Nutrition and Human Health Wellington, New Zealand Werner Kunz Institute of Physical and Theoretical Chemistry University of Regensburg Regensburg, Germany 89595_C000.indd xxix 89595_C000.indd xxix 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
xxx Contributors Silvia Lucangioli Analytical Chemistry and Physicochemistry Department Faculty of Pharmacy and Biochemistry University of Buenos Aires Buenos Aires, Argentina S. K. Mehta Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India J. C. Mejuto Department of Physical Chemistry Faculty of Sciences University of Vigo at Ourense Ourense, Spain Satya Priya Moulik Center for Surface Science Department of Chemistry Jadavpur University Kolkata, West Bengal, India Nashaat N. Nassar Department of Chemical and Petroleum Engineering University of Calgary Calgary, Alberta, Canada V. Papadimitriou Institute of Biological Research and Biotechnology National Hellenic Research Foundation Athens, Greece Animesh Kumar Rakshit Department of Natural Sciences West Bengal University of Technology Kolkata, West Bengal, India André Richardt Armed Forces Scientific Institute for NBC Protection Munster, Germany Nilmoni Sarkar Department of Chemistry Indian Institute of Technology Kharagpur, West Bengal, India Debabrata Seth Department of Chemistry Indian Institute of Technology Kharagpur, West Bengal, India H. Stamatis Biological Applications and Technologies Department University of Ioannina Ioannina, Greece C. Tojo Department of Physical Chemistry Faculty of Chemistry University of Vigo Vigo, Spain Matija Tomšič Faculty of Chemistry and Chemical Technology University of Ljubljana Ljubljana, Slovenia Didier Touraud Institute of Physical and Theoretical Chemistry University of Regensburg Regensburg, Germany Valeria Tripodi Analytical Chemistry and Physicochemistry Department Faculty of Pharmacy and Biochemistry University of Buenos Aires Buenos Aires, Argentina 89595_C000.indd xxx 89595_C000.indd xxx 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
Contributors xxxi Stefan Wellert Physical Chemistry I University of Bayreuth Bayreuth, Germany A. Xenakis Institute of Biological Research and Biotechnology National Hellenic Research Foundation Athens, Greece Anan Yaghmur Institute of Biophysics and Nanosystems Research Austrian Academy of Sciences Graz, Austria 89595_C000.indd xxxi 89595_C000.indd xxxi 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
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1 1 A Phase Diagram Approach to Microemulsions Stig E. Friberg and Patricia A. Aikens CONTENTS 1.1 Introduction..................................................................................................1 1.2 Discussion ....................................................................................................2 1.2.1 Ordering–Disordering.......................................................................2 1.2.2 Temperature Dependence .................................................................7 1.2.3 Vapor Composition from Microemulsions ..................................... 10 1.3 Conclusion.................................................................................................. 14 Symbols and Terminologies................................................................................ 14 References........................................................................................................... 14 1.1 INTRODUCTION The phase diagram approach to microemulsions was introduced decades ago by Gillberg and collaborators [1]. At that time, it was not well received by the researchers in the area, because it emphasized that microemulsions are in fact micellar systems and the traditionally simplified thermodynamic treatment was very much in vogue at that time. Unfortunately, the ensuing arguments about the “true structure of microemulsions” shrouded the advantage of the approach, and it was only after the Israelachvili–Ninham analysis of the thermodynamics of such systems [2] that attention could be directed to the essential features of the phase diagram approach. A brief history of the development has been given by Lindman and Friberg [3]. In the following sections, the phase diagram approach will be applied to three attributes of microemulsions: (a) the importance of ordering versus disordering, (b) the temperature dependence of the behavior of microemulsions stabilized by polyethylene glycol adduct surfactants, and (c) the use of phase diagrams to obtain information on the composition of the vapor leaving microemulsion during its evaporation. 89595_C001.indd 1 89595_C001.indd 1 11/3/2008 10:21:55 AM 11/3/2008 10:21:55 AM
2 Microemulsions: Properties and Applications 1.2 DISCUSSION 1.2.1 ORDERING–DISORDERING The phase diagrams of microemulsions have traditionally been presented in two ways. The original one was built on the results of Ekwall on the association structures of amphiphilic systems [4] and was based on the associations in the water–surfactant combination. According to this approach, the development of the microemulsion structures was a result of the structural modifications brought about by the addition of less hydrophilic amphiphiles such as alcohols. The hydro- carbons in the microemulsions were considered solubilizates in this methodology and their effect on the structure was considered to be of secondary importance. The approach was very successful for W/O microemulsions, providing a simple tool for their formulation. A generic diagram is given in Figure 1.1. The essential feature of importance for the microemulsion is the fact that the inverse micellar solution and the aqueous solution of normal micelles are not in mutual equilibrium except for extremely low-surfactant concentrations. For higher-surfactant concentrations, the equilibrium is with the liquid crystalline phase. As a consequence, the transition from the normal micelles to inverse micelles (Figure 1.2) does not happen directly, but through a lamellar liquid crystal (Figure 1.3). W/O microemulsions stabilized by an ionic surfactant also employ a less hydrophilic amphiphile, which is known as the cosurfactant. The original cosur- factants were alcohols [5] and Gillberg realized early on [1] that W/O microemul- sions were obtained simply by adding a hydrocarbon to Ekwall’s inverse micellar solution (Figure 1.4). Addition of the hydrocarbon does not imply significant FIGURE 1.1 Partial generic phase diagram of a system water (W), surfactant (S), and medium chain length alcohol (A). (Adapted from Ekwall, P., in Advances in Liquid Crystals, Brown, G.H. (Ed.), Academic Press, New York, 1975, pp. 1–139. With permission.) A S W MS LLC IMS 89595_C001.indd 2 89595_C001.indd 2 11/3/2008 10:21:56 AM 11/3/2008 10:21:56 AM
A Phase Diagram Approach to Microemulsions 3 FIGURE 1.2 In the aqueous solution micelle (left), the surfactant polar groups are orga- nized toward the surrounding water, while the hydrocarbon chains are inside the micelle. In an inverse micelle (right), the organization is opposite. Water Aqueous micelle Inverse micelle FIGURE 1.3 In a lamellar liquid crystal, water layers are separated by mirrored bilayers of surfactant. FIGURE 1.4 Addition of hydrocarbon to the inverse micellar solution (solid line) (Figure 1.1) gives a W/O microemulsion (hatched line). A/H S W 89595_C001.indd 3 89595_C001.indd 3 11/3/2008 10:21:56 AM 11/3/2008 10:21:56 AM
4 Microemulsions: Properties and Applications structure changes [6] and the W/O microemulsions were hence described as inverse micellar solutions. The approach was initially not received well by Schulman’s successors [7], and it is remarkable that Schulman’s initial publica- tion on the concept described these microemulsions as colloid solutions. The term “microemulsion” was coined much later [8]. The application of Ekwall’s presentation of phase diagrams offers several advantages. First, it provides an explanation of the fact that when the capacity to include water in a W/O microemulsion is exceeded, the phase appearing is not an aqueous liquid, but a lamellar liquid crystal. Secondly, it provides immediate clar- ification of the role of the cosurfactant. As demonstrated in Figure 1.5, the effec- tiveness of the cosurfactant depends decisively on its chain length. The difference in the sizes of the W/O microemulsion regions in Figure 1.5 demonstrates that decanol is far less useful as a cosurfactant than pentanol (if it is even useful at all). The explanation for this fact is not, as it may appear at a first glance, the difference in the stability of the inverse micelles; it rests with the fact that the shorter pentanol chain destabilizes the lamellar liquid crystal by disordering it, and so as a result, increasing the area for the inverse micellar solution. Following this approach, it would be logical to use butanol as a cosurfactant instead of pentanol, because its isotropic liquid region now expands continuously to the water corner (Figure 1.6). This large continuous isotropic liquid region at first appears highly appealing, but effective utilization of shorter chain length alcohols as cosurfactants is coun- tered by another factor. Butanol certainly destabilizes the lamellar liquid crystal efficiently (Figure 1.6), but when the hydrocarbon is added to form the micro- emulsion, the butanol is too water soluble and does not reach and reside at the oil/ water interface sufficiently. As a result, the system forms two separate phases: a traditional macroemulsion of oil and water. C5OH C10OH CnOH S W FIGURE 1.5 Comparison of the inverse micellar liquid areas for systems with pentanol and decanol. 89595_C001.indd 4 89595_C001.indd 4 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
A Phase Diagram Approach to Microemulsions 5 The importance of the disordering action of the cosurfactant is confirmed by a later publication concerning O/W microemulsions [9]. In this case, the pentanol per se did not provide sufficient disordering effect as demonstrated by the features in Figure 1.7. The insufficient disordering is illustrated by the fact that the decane solubili- zation is limited and by the solubility gap along the sodium dodecyl sulfate (SDS)/W–C5OH axis. The latter is caused by the lamellar liquid crystal between the aqueous and pentanol solution (Figure 1.8). The addition of a hydrotrope, a more water soluble molecule with disordering action, supplemented the disordering and the liquid crystal range along the SDS/ W–C5OH axis disappeared (Figure 1.9) resulting in an excellent microemulsion area [10]. FIGURE 1.6 Isotropic liquid area for a system with butanol. C4OH Isotropic liquid S W FIGURE 1.7 Isotropic liquid in the partial phase diagram of decane, n-C10, pentanol, C5OH, and a solution of 15% SDS, in water. The liquid structure passes from an O/W microemulsion to a W/O one through a bicontinuous structure without a phase separation. SDS/W 15/85 I II O/W microemulsion W/O microemulsion Bicontinuous microemulsion N-C10 n-C5OH 89595_C001.indd 5 89595_C001.indd 5 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
6 Microemulsions: Properties and Applications The surfactant in this system is ionic, and hence salt has a similar action [11]. The ultimate extension of this action is amply exemplified in the early publications from the field of microemulsion-assisted petroleum recovery [12]. The phase diagram approach to microemulsions following Ekwall [1] is characterized by a section through the three-dimensional diagram according to Figure 1.10a. Alternative publications with different sectioning (Figure 1.10b) have also gained popularity [13]. Both these presentations are useful; the second one suffers from the disadvantage of not catching the strong variation in the areas with the surfactant/cosurfactant ratio as accentuated by Figure 1.8. FIGURE 1.8 Part of the phase diagram water (W), SDS, and pentanol (C5OH). The areas named microemulsions in this Figure 1.4 were called micellar solutions in Ekwall’s terminology (From Ekwall, P., in Advances in Liquid Crystals, Brown, G.H. (Ed.), Academic Press, New York, 1975, pp. 1–139. With permission.) SDS W Bicontinuous microemulsion Lamellar liquid crystal C5OH W/O microemulsion O/W microemulsion FIGURE 1.9 Microemulsion region in the system water/SDS/sodium xylene sulfonate, W/SDS/SXS, pentanol, C5OH, and decane, C10. C5OH C10 W/SDS/SXS 80.9/14.3/4.8 89595_C001.indd 6 89595_C001.indd 6 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
A Phase Diagram Approach to Microemulsions 7 1.2.2 TEMPERATURE DEPENDENCE It is seen above that the areas for microemulsions stabilized by ionic surfactants are decisively dependent on the structure of the cosurfactant to cause the necessary disorder in the system. Microemulsions stabilized by polyethylene glycol adduct nonionic surfactants, on the other hand, are characterized by the fact that cosurfactant is not used. Instead, the areas of stability now rely on temperature (Figure 1.11) although the relation with the liquid crystal structure is still the essential element [14]. The main theme of this dependence is illustrated in Figures 1.12 and 1.13 [15], which show the generic phase diagram for an alkyl ether surfactant with an aliphatic hydrocarbon of a moderate length (approx. 12 carbons), and a short polyethylene glycol chain (approx. 4 ethylene glycol units). First, the diagram is characterized by a complete disparity of the solubility of the surfactant in water FIGURE 1.10 Two main representations of the microemulsion pseudophase diagram. The left depiction (a) is the Ekwall–Gillberg approach, which treats the hydrocarbon/ cosurfactant liquid as one component, while the right model (b) combines the surfactant and cosurfactant into one component. W S Co-S H W S Co-S H (b) (a) FIGURE 1.11 Phase equilibria for the system water (W), a polyethylene glycolalkyl ether (S) and an aliphatic hydrocarbon (H). Low-temperature behavior is depicted in the upper left-hand diagram, high-temperature features are depicted in the lower right-hand diagram. W S H S W H 89595_C001.indd 7 89595_C001.indd 7 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
8 Microemulsions: Properties and Applications and hydrocarbon with a moderate rise in temperature to 75°C.At lower temperature, the solubility is restricted to water transitioning into an aqueous micellar solution, followed by a lamellar liquid crystal, and finally an inverse micellar liquid with increasing surfactant content. At a higher temperature, the surfactant solubility is restricted entirely to the hydrocarbon, without any ordered structures. W H.S. H FIGURE 1.12 Features of the system at the HLB temperature. FIGURE 1.13 Three next stages after the state at left in Figure 1.11. S H W O/W microemulsion W Internal two-phase W 89595_C001.indd 8 89595_C001.indd 8 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
A Phase Diagram Approach to Microemulsions 9 In between these two temperature extremes, lies the hydrophilic–lipophilic balance (HLB) temperature. Here, the aqueous micellar phase has disappeared or is severely restricted (Figure 1.13), and a liquid phase is found instead at a water/ hydrocarbon ratio of approximately 1 with a moderate concentration of surfactant. This liquid is a microemulsion with a bicontinuous structure. From a phase diagram point of view, the development of the features from those in the left-hand diagram of Figure 1.11 to those in Figure 1.12 is of interest because of the rather intricate details of the equilibria. Figure 1.13 presents the initial changes in greater detail. The first development from the stage in the left-hand diagram of Figure 1.11 is that the hydrocarbon and the surfactant become mutually soluble and the lamellar liquid crystal is extended toward high-hydrocarbon content. These two areas remain approximately constant during the next stages. In the first of these (top right, Figure 1.13), the micellar region along the water–surfactant axis is limited with respect to the maximum water content. To reach the water corner, a certain ratio of hydrocarbon to surfactant is required. The effect is that a two-phase region is formed extending from the water corner. In the next step (bottom right, Figure 1.13), the two regions are separated and the one with highest hydrocarbon to surfactant ratio forms an O/W microemulsion, which has the specific property of being infinitely dilutable with water. It should be noted that the O/W microemulsions stabilized by an ionic surfactant such as the one in Figure 1.9 cannot be diluted with water. Any such attempt leads to phase separation and a macroemulsion is formed. The fact that there is no equilibrium between the O/W microemulsion and the remnant of the micellar solution is a remarkable feature in the diagram. Instead, the two liquids are in equilibrium with pure water and with the lamellar liquid crystal. Further progression toward the HLB temperature pattern in Figure 1.12 is depicted in Figure 1.14. Now the O/W microemulsion area is separated from the water corner, but forms an unconnected phase in the water-rich part of the system. The ensuing phase equilibria are depicted in the enlarged partial diagram on the left-hand side of Figure 1.14. The complexity of the equilibrium conditions are well illustrated by the FIGURE 1.14 Subsequent step to the configurations in Figure 1.13. S H W W 89595_C001.indd 9 89595_C001.indd 9 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
10 Microemulsions: Properties and Applications number of phases (MS, aqueous micellar solution; O/W, O/W microemulsion; LC, liquid crystal; O, hydrocarbon–surfactant liquid) found when adding hydrocarbon to an aqueous solution of surfactant less concentrated than the separated phase. The sequence isW + MS →W + MS + LC →W + LC →W + LC + O/W →W + O/W → W + O/W + O→W + O; an extraordinary number of seven combinations for surfactant concentration less than that of any association structure phase. The bicontinuous microemulsion region coalesces with the hydrocarbon–sur- factant liquid area forming a W/O microemulsion region reaching toward greater fractions of water. The surfactant–hydrocarbon ratio for maximum water solubiliza- tion depends on the maximum solubilization of the hydrocarbon into the lamellar liquid crystal. With increasing temperature, the solubilization capacity is reduced and the maximum for water solubilization into the W/O microemulsion is shifted to greater surfactant–hydrocarbon ratios. The final result of this trend is the system of the hydrocarbon–surfactant liquid in equilibrium with water, in accordance with the right part of Figure 1.11 (Figure 1.15E). 1.2.3 VAPOR COMPOSITION FROM MICROEMULSIONS The recently introduced algebraic method to extract information from phase diagrams [16] has been used to quantify evaporation from microemulsions [17]. The approach as such does not provide additional information to the experimentally determined phase diagram, but introduces a system to illustrate the influence of the relative humidity (RH) on the direction and volume of the evaporation and its path that is not immediately available otherwise. H W H.S. A H W H.S. C H W H.S. B H W H.S. E H W H.S. D FIGURE 1.15 Subsequent patterns between that of the HLB temperature in Figure 1.12 and the final appearance to the right in Figure 1.11. (Reproduced from Shinoda, K. and Friberg, S.E., Emulsions and Solubilization, John Wiley & Sons, New York, 1986.) 89595_C001.indd 10 89595_C001.indd 10 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
A Phase Diagram Approach to Microemulsions 11 The method basically relates the changes in the composition of the liquid phase to the composition of the released vapor. This information would not be of practical value if the evaporation took place in completely dry air or in vacuum; the difference in the composition of the liquid phase obviously equals the compo- sition of the escaping vapor. In addition, if the evaporation were to take place under conditions close to equilibrium, the partial vapor pressures could be utilized to calculate the activities of the components in the liquid phase. Evaporation how- ever, usually takes place into an atmosphere at a certain level of humidity and this fact affects the interaction between the liquid phase and its vapor. This influence is conveniently attended to using the algebraic approach as illustrated by an example [17]. The system to be discussed consists of water (W), cosurfactant (C), and surfactant (S) and is depicted in Figure 1.16. The composition of the discharged vapor from a weight fraction composition (W1, C1, and S1) is obtained from the tangent to the experimentally determined evaporation path, which is given a function C(S). 1 1 1 1 ) ( d /d C C S S C S = + − (1.1) Setting S = 0, the composition of the released vapor (WV, CV, and 0) is found. V 1 1 1 1 ( / ) d d C C S C S = − (1.2) and V 1 1 1 1 1 d / ) d ( W C S C S = − + (1.3) FIGURE 1.16 System used is the W/O microemulsion base in which the cosurfactant is a volatile compound, illustrating the behavior of a hydrocarbon. W S C Evaporation composition (W1, C1, and S1) Composition of released vapor (Wv, 0, and Sv) 89595_C001.indd 11 89595_C001.indd 11 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
12 Microemulsions: Properties and Applications This is the composition of the released vapor as obtained from experimental results. These are inherently affected by the RH of the receiving atmosphere with the actual vapor phase including the indigenous atmospheric water (WRH). V 1 1 1 1 RH 1 d / ( ) d W C S C S W = − + + (1.4) Realizing that the contribution from the released microemulsion vapor to the total water in the vapor is proportional to its evaporation rate and assuming ideal behavior of the vapor, the weight fractions of the two volatile compounds in the released vapor become V C C C C W W W { ( /[ Em) 0.01RH 0)} ( ] C P M P M M P P = + − μ (1.5) and V W W W C C W W W { ( ( { ( Em) 0)}/[ Em) 0.01RH 0)}] ( W M P P P M M P P = μ − + μ − (1.6) where P is pressure M is molecular weight As a contrast the equilibrium values for the receiving atmosphere are V C C C C W W /[ ( Em)] C P M P M M P = + μ (1.7) and V W W C C W W Em)/[ ( Em)] ( W M P P M M P = μ + μ (1.8) The combination of these equations with the phase diagram offers a great deal of insight on the evaporation [17]. A few salient points will be examined here. At first, the indigenous vapor pressure of water in the atmosphere will exceed the vapor pressure from the microemulsion for sufficiently high values of RH. As a consequence, the evaporation of water is now reversed; it is absorbed into the microemulsion liquid, as quantified by the general relation of the ratio between evaporating water and volatile organic compound, a first-order linear equation: C C W W W C C { ( ( [ Em) 0.01RH 0)}]/ R P M M P P P M = + μ − (1.9) The reversal of direction happens when the RH exceeds the value obtained in Equation 1.10: W W RH Em)/0.01 0 ( ) ( P P > μ (1.10) Figure 1.17 illustrates this change of course in the phase diagram (negative CV). 89595_C001.indd 12 89595_C001.indd 12 11/3/2008 10:22:00 AM 11/3/2008 10:22:00 AM
A Phase Diagram Approach to Microemulsions 13 With realistic values of the vapor pressures and the molecular weights, such as PC = 0.1 PW(0) = 20 PW(μEm) = 10 MW = 18 MC = 150 Equation 1.9 becomes / 12 0.24RH W C = − (1.11) and the reversal takes place at an RH of 50%. The RH has another critical point representing the value at which the reduction in the entire weight of the microe- mulsion with evaporation is reversed. The general expression is W W C C W W RH ( Em) /0.01 ( ) 0) ( M P P M M P = μ + (1.12) This represents a slightly greater value than that for the reversal of the water transport direction in Equation 1.10 with a numerical value of the RH in this case W S C (W1, C1, S1) Composition of released vapor (Wv, 0, and Sv) x Water evaporating Water absorbed No water evaporation FIGURE 1.17 Phase diagram illustrating the change in water evaporation direction (arrows from the evaporation composition, W1, C1, and S1) with increased RH. The evapo- ration changes from water evaporating to water being absorbed into the liquid from the atmosphere. 89595_C001.indd 13 89595_C001.indd 13 11/3/2008 10:22:03 AM 11/3/2008 10:22:03 AM
14 Microemulsions: Properties and Applications being 54%. In what may be considered an oddity, this value is also the one at which the value of CV for the released vapor instantaneously goes from positive to negative infinity with increased RH. 1.3 CONCLUSION Phase diagrams are shown to provide valuable information on the role that struc- ture of the surfactant, cosurfactant, and oil plays in determining the properties of the system at any composition. In addition, it is demonstrated that degree of order/ disorder of lamellar liquid crystalline phases within a system stabilized by an ionic surfactant is determined by the chemical structure of the components and this in turn influences the magnitude, the nature (O/W, bicontinuous, W/O), and the location of the microemulsion phases. When nonionic polyethylene glycol ether surfactants are used instead of ionic ones, the temperature of the system is the crucial determining factor, with optimum properties at the HLB temperature of the surfactant. Finally, it is shown that straightforward algebraic analysis can be used to determine the composition of the evaporating gas from the microemulsion under practical use conditions by taking into account the atmospheric RH, utilizing the vapor pressures of the volatile components, molecular weights, and extrapolations from the phase diagram. SYMBOLS AND TERMINOLOGIES W water O oil C cosurfactant H hydrocarbon A alcohol μEm microemulsion RH relative humidity P pressure M molecular weight REFERENCES 1. Gillberg, G., Lehtinen, H., and Friberg, S. E. (1970). IR and NMR investigation of the conditions determining the stability of microemulsions, J. Colloid Interf. Sci. 33, 40–49. 2. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W. (1976). Thermodynamics of amphiphilic association structures, J. Chem. Soc. Faraday Trans. II 72, 1525–1533. 3. Lindman, B. and Friberg, S. E. (1999). Microemulsions—a historical overview, in Mittal, K., Kumar, P. (Eds), Handbook of Microemulsions Science and Technology, Marcel Dekker, New York, pp. 1–12. 4. Ekwall, P. (1975). Composition, properties and structures of liquid crystalline phases in systems of amphiphilic compounds, in Brown, G. H. (Ed), Advances in Liquid Crystals, Academic Press, New York, pp. 1–139. 89595_C001.indd 14 89595_C001.indd 14 11/3/2008 10:22:04 AM 11/3/2008 10:22:04 AM
A Phase Diagram Approach to Microemulsions 15 5. Hoar, T. P. and Schulman, J. H. (1943). Transparent water-in-oil dispersions: The oleopathic hydro-micelle, Nature 152, 102–103. 6. Sjoeblom, E. and Friberg, S. (1978). Light-scattering and electron microscopy deter- minations of association structures in W/O microemulsions, J. Colloid Interf. Sci. 67, 16–30. 7. Prince, L. M. (1969). A theory of aqueous emulsions II: Mechanism of film curva- ture at the oil/water interface, J. Colloid Interf. Sci. 29, 216–221. 8. Schulman, J. H., Stoeckenius, W., and Prince, L. M. (1959). Mechanism of forma- tion and structure of micro-emulsions by electron microscopy, J. Phys. Chem. 63, 1677–1680. 9. Friberg, S. E., Brancewicz, C., and Morrison, D. (1994). O/W microemulsions and hydrotropes: The coupling action of a hydrotrope, Langmuir 10, 2945–2949. 10. Friberg, S. E. (1997). Hydrotropes, Curr. Opin. Surf. Colloid Sci. 2, 490–494. 11. Friberg, S. E. and Buraczewska, I. (1978). Microemulsions in the water–potassium oleate–benzene system, Prog. Colloid Polym. Sci. 63, 1–9. 12. Bourrel, M. and Schechter, R. S. (1988). Microemulsions and Related Systems, Marcel Dekker, New York. 13. Bauduin, P., Touraud, D., and Kunz, W. (2005). Design of low-toxic and tempera- ture-sensitive anionic microemulsions using short propyleneglycol alkyl ethers as cosurfactants, Langmuir 21, 8138–8145. 14. Stubenrauch, C., Frank, C., Strey, R., Burgemeister, D., and Schmidt, C. (2002). Lyotropic mesophases next to next to highly efficient microemulsions: A 2H NMR study, Langmuir 18, 5027–5039. 15. Shinoda, K. and Friberg, S. E. (1986). Emulsions and Solubilization, John Wiley & Sons, New York. 16. Friberg, S. E. (2006). Weight fractions in three-phase emulsions with an Lα phase, Colloids Surf. A. 282/283, 369–376. 17. Friberg, S. E. and Aikens, P. A. submitted to J. Disp. Sci. Tech. 89595_C001.indd 15 89595_C001.indd 15 11/3/2008 10:22:04 AM 11/3/2008 10:22:04 AM
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17 2 Physicochemistry of W/O Microemulsions: Formation, Stability, and Droplet Clustering Animesh Kumar Rakshit and Satya Priya Moulik CONTENTS 2.1 Introduction................................................................................................ 17 2.2 Basics of Formation ................................................................................... 21 2.3 Stability of Microemulsion ........................................................................ 22 2.3.1 Method ......................................................................................... 22 2.3.2 Droplet Dimensions ..................................................................... 32 2.3.3 Energetics of Droplet Clustering .................................................34 2.3.4 Dynamics of Dispersed Droplets................................................. 36 2.3.5 Conductance of Microemulsions ................................................. 36 2.3.6 Useful Percolation Equations....................................................... 39 2.3.7 Conductance Percolation and EMT ............................................. 41 2.3.8 Viscosity Percolation ................................................................... 43 2.3.9 Antipercolation and Double Percolation......................................44 2.3.10 Activation Energy for Percolation................................................ 45 2.3.11 Additive Effects on Percolation....................................................46 Acknowledgments............................................................................................... 49 Symbols and Terminologies................................................................................ 49 References........................................................................................................... 51 2.1 INTRODUCTION Microemulsions are microheterogeneous, thermodynamically stable mixtures of oil and water. Here, the term “oil” means any water insoluble organic liquid. Macroscopically, they are homogeneous systems. Such oil/water disperse sys- tems were known for a long time as there were some commercial floor cleaning products available in the American market at the turn of the twentieth century. 89595_C002.indd 17 89595_C002.indd 17 11/3/2008 10:32:20 AM 11/3/2008 10:32:20 AM
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some of the big ones.” “I think I can get another like yours right here at shore,” said Tom, and he threw in. Shortly he had a bite, and almost duplicated Jack’s catch. Meanwhile Jack was cleaning and scaling his prize, and drying himself out. The other boys had fair luck with rod and line, and then it was up to Dick to cook the fish, which he did, frying them in bacon and corn meal. “Oh, say, maybe they don’t smell good!” cried Tom, as the savory odor was spread about the camp. “They’ll taste better,” was Jack’s comment. The evening meal was a great success and they all voted that Dick was a much better cook than he had given himself credit for. “How are you on pies?” asked Tom, as they sat around the campfire that evening, after everything was ready for bed. “Think you can tackle them, Dick? We’ve got prepared flour, and you can use some jam, or canned apples, for filling.” “I’ll try it,” agreed the amateur cook. “We’ll have pie to-morrow.” They did not sleep very well that night, as the beds were rather hard, not having been properly made, and they were all rather excited over the events of the day. Breakfast, however, with coffee, and bacon and eggs which they had brought from Wilden, put them all in good humor, and they made a merry meal. “Now for some fishing!” exclaimed Tom, as he went down to look about his motorboat. “And I’m going to take a gun and see if I can get anything in the line of game,” put in Jack. “It’s out of season for most things, but I may get something in the bird line.”
“And Dick is going to make pie,” said Bert. “Make four, old man, so there’ll be one apiece.” “All right,” agreed the young cook good naturedly. “I won’t guarantee results, but I’ll do my best.” Tom started out in the boat with Bert to do some fishing, while Jack wandered off in the woods with his shotgun. Dick did up the dishes and then began rummaging around in the supplies. Soon he was whistling away and, as Tom and Bert could see, from where they were in the boat, he was kept quite busy over something. “Well, did you get ’em made?” asked Tom, when they had all assembled for dinner. “How about the pies, Dick?” “There they are,” was the retort, and Dick pointed to the pastry. “Hum! They smell good!” exclaimed Jack, as he whiffed an odor from the pies. “They look good,” commented Tom. “Let’s see if they taste good,” suggested Bert. The pies were served as dessert, and at the first mouthful Tom let out a howl. “For the love of tripe!” he cried. “What did you put in these pies, Dick?” “Apples, of course,” replied the injured cook. “What did you suppose it was?” “Well, if those are apples then they’re flavored with something funny,” declared Tom. “Where’s the can you used?” Dick brought two empty tin cans up to the table, which was made from packing boxes. “The paper labels soaked off,” he explained, “but there were cans of apples on top and below these so I thought it was all right. Isn’t it?” Tom took a smell, and cried out:
“Say, fellows, he dumped a can of quinces in the apple pie stuff and baked that all together and then used baking soda for powdered sugar! Oh wow! What a taste!” There was a general laugh, and Dick replied with: “Well, if you fellows think you can do any better you can have my job. I’m sick of being cook.” “Tut, tut! It’s all right,” said Tom hastily. “We were only fooling. You’re doing fine, Dick, only, after this, smell of a can if it hasn’t got a label pasted on it, and taste the powdered sugar.” But if the pies were a failure, the rest of the dinner was good, and later on Dick proved that he could make good pastry when he used the right ingredients. They had more fish that day, as luck was good, but the game was scarce, as might have been expected at that season of the year. After dinner, the rest of the day was spent in getting the camp into better shape, and making the beds more comfortable. The boys were in the habit of making up a camp fire early in the evening, and sitting in the glow of it to talk. They did this on their second night, and when it had about died down Tom tossed on some heavy sticks of wood and remarked: “Well, I’m going to turn in. I’m tired and I want some sleep. To- morrow we’ll take a long boat ride.” “When are we going to the old mill?” asked Jack. “Oh, maybe we can try that soon if we like,” said Tom. It was nearly midnight, as Tom ascertained by looking at his watch, when he was suddenly awakened by hearing something moving about near the sleeping tent. At first he thought it was one of his chums, and he called out: “Who’s that? You, Jack?”
There was no answer, and, looking across to the other cots, our hero saw the forms of his companions under the covers. They were all quiet. “There’s some one out there,” he murmured. Rising cautiously he stepped to the flap of the canvas shelter and peered out. In the dying glow of the camp fire he saw an old man silently walking toward the tents. “For gracious sake!” breathed Tom to himself. “If that isn’t the old hermit of the mill I’m a lobster! I wonder what he’s doing here?” With anxious eyes he watched, and as the moon came out from behind a cloud, to add to the glow of the campfire, Tom saw the light glint on a gun. “He’s looking for us!” whispered Tom. “I wonder what I’d better do?”
CHAPTER VIII OLD ACQUAINTANCES For a moment the lad stood there at the flap of the tent, pondering over the situation. He realized that he might have a desperate character to deal with—a man who would not listen to reason, and who was impulsive, as evidenced by his leap into the water after the motorboat. “But I’ve got to do something,” thought Tom. “If I don’t he may take a shot at us, not meaning to do any harm, but just because he’s erratic. And that sort of a bullet does just as much harm as any other. If he should fire into the tent——” Tom did not finish out his thought, for at that moment there was a movement on the part of the old man. He had been standing still, silently regarding the camp, and now he again advanced. “He’s going to see what sort of a place we have here,” mused Tom. “I wonder if I’d better awaken the boys?” He thought it over for a moment and then decided that perhaps he could best deal with the old man alone. “But how?” he asked himself. Tom watched the hermit. He came on with a tread like that of a cat—silently—stealthily—peering from side to side. At times he muttered to himself. “I’ll see if I can take him by surprise,” decided Tom. Stepping back, where he could not be seen, inside the tent, our hero suddenly yelled:
“Get out of here! What are you doing in our camp? Be off before I set the dogs on you!” The old man was evidently startled. He stiffened as he stood, but Tom was glad to see that he did not bring the gun to bear. From under the shaggy eyebrows the hermit gazed about him as if to determine whence came the voice. But if Tom had any idea that he could frighten the man into going away he was mistaken. For the hermit of the mill came forward until he stood directly in front of the big tent, and then, straightening up, he fairly shouted: “Ha! I have found you; have I? Those who brought their infernal puffing engine on my lake. Now you are in my woods. I have been looking for you. I warn you away! You must leave at once! I will not be cheated out of my fortune this way. Leave my woods or it will be the worse for you!” and he shook his fist at Tom, who had now stepped into view at the flap of the tent. “Hello! What’s the row?” called Jack, suddenly awakening. “Is the camp on fire?” asked Dick. “What’s wrong, Tom?” cried Bert, and all three of our hero’s chums sprang from their cots and crowded around him. “It’s our old friend the hermit of the mill,” explained Tom in a low voice. “He’s come to drive us out of the woods.” “What are you going to do?” asked Jack. “I don’t know. Let’s see what he does.” “He may be dangerous,” commented Dick. “And these may be his woods,” added Bert. “Nonsense,” declared Tom. “I asked dad about it before I came up, and he said this part of the forest belonged to a big lumber company that was holding it for the trees to get bigger before cutting. This old man doesn’t own it any more than we do.”
“Then you’re going to stick?” “I sure am!” During this talk the old hermit remained motionless, regarding the boys with angry eyes. Then he spoke again. “Well, are you going to take yourselves out of my woods? Are you going to leave at once? I demand that you go!” “No, sir, we are not going,” declared Tom, firmly but respectfully, for after all, he thought the age of the man was entitled to some deference. “You must leave my woods!” the hermit insisted. “I have been bothered enough in the search for the fortune hidden from me. I want to be alone in my woods. Go!” and he pointed his finger toward Wilden. “I do not think you have the right to make us go,” said Tom. “I understand these are not your woods, and we have as good a right to camp here as you have to wander about. We are not going!” For a few seconds the old man seemed dazed at the bold answer. Probably he had expected a meek compliance, but, as it developed, Tom’s answer was the best that could have been given. Pausing a moment the hermit gazed almost reproachfully at the lads and then, with another shake of his fist at them, he called: “Well, you have been warned, and now you must take the consequences. The price of your folly is on your own heads!” He turned and vanished into the shadows of the woods. “Whew! Quite dramatic!” exclaimed Tom, as he turned to his chums. “I should say so,” agreed Jack. “Nice thing to be awakened from pleasant dreams and told to move on in a trackless forest at midnight. He’s as bad as Professor Skeel used to be.”
“Speaking of Skeel reminds me,” observed Tom. “Do you think he has come up here to camp?” “Hard to say,” murmured Bert. “But I know one thing, as long as I’m awake I’m going to have something to eat. Are there any of those chicken sandwiches left, Dick?” “I guess so. And there’s some cold tea.” “Warm it up then, and we’ll have a lunch.” “Say, what do you think this is; a quick-eat restaurant?” asked the amateur cook. “Oh, go ahead,” suggested Tom. “We’ll all help, and maybe we’ll get to sleep again, after this interruption, if we eat.” The oil stove was lighted, and the tea put on to warm, while Dick set out a plate of sandwiches he had made from canned chicken. Then the boys ate and talked. “That old hermit is sure on our trail,” declared Tom. “But he doesn’t seem to be as dangerous as the folks made out,” commented Jack. “I guess he’s just simple-minded, thinking of the treasure in the old mill,” added Bert. “By the way, Tom, when are we going to visit the ruins, and have a try for the buried gold?” and he laughed. “Oh, we’ll go over there some time,” agreed Tom. “I’d like to pick a day, though, when old Wallace wouldn’t be on hand. I’m not exactly afraid of him, but, from what I can understand, he does own the mill, though not these woods, and if he ordered us off that property we’d have to go.” “But we can take a chance,” suggested Dick. “Oh, sure,” came from Tom. “Say, but that old chap must spend all his time wandering about the woods. I wonder where he sleeps when he’s away from the mill?”
“Oh, he probably has plenty of bunks and caves that we never would dream of,” said Jack. “Well, I’m going to turn in,” he added, with a yawn. “If he comes back again kindly tell him, Tom, to wait until morning before doing any more ordering-off.” Once more the lads sought their cots, to sleep undisturbed until morning. The day was spent in getting their camp more in ship- shape, and getting in a supply of wood for camp fires, and for cooking in case their oil gave out, or the portable stove failed. In the afternoon they went fishing, and had good luck. Though they kept watch for the hermit, they did not see him. The woods and lake were as deserted as though they were in some country as yet unvisited by man, and there were no evidences that any camping parties had ever visited the region where the boys were. “It sure is wild,” said Jack, as he gazed about. “It’s just the cheese though,” declared Tom. “We couldn’t have picked out a better place.” “And as soon as we get busy on the secret of the old mill there may be lots of happenings,” added Bert. A week passed, during which our friends enjoyed life to the utmost. They fished, and as the lake had seldom been visited by devotees of the rod and line it proved a garden spot for such sport. One had but to throw in a line to have a bite. They hunted, too, but as the season was not open they managed to kill only a few foxes and skunks, and, as their fur was not of much value in the summer, even this they gave up as rather unprofitable work. “It’s the mill we want to head for,” insisted Jack. “Come on, Tom, let’s get up an expedition and go there. We can go in the boat, for, as you say, the mill is on the river that runs into the lake. Come on.” “All right, we’ll go to-morrow,” agreed Tom. Accordingly, having set their camp to rights, and having put up a lunch, for they would not be back to dinner, they set off in the Tag, heading up the lake to where the river entered it.
“She’s running better than she did at home,” remarked Dick to Tom, as he looked at the puffing motor. “Yes, but don’t say anything,” cautioned our hero. “She may be holding back for a kick-up. I never praise this motor, for I actually believe it knows what you say. Let well enough alone,” and the others laughed at his quaint conceit. It was a beautiful day, and the trip along the lake was much enjoyed. It was rather lonesome, but the boys did not mind that. As they moved along the shore of a little cove Jack suddenly called: “Hold on! I think I heard something moving near the bank there,” and he pointed just ahead. “Slow down the engine,” called Tom to Dick, and the latter throttled down, making the machinery almost noiseless. Then they all heard a crashing in the underbrush. “Maybe it’s the hermit,” suggested Bert. “Very likely,” agreed Jack. “I hope he doesn’t begin on one of his tantrums again.” The sounds in the bushes grew, and a moment later three figures suddenly stepped into view on the sandy beach of the lake. “Look!” exclaimed Tom in a low voice. “If this isn’t the limit!” All four boys gazed toward the figures, to behold their old acquaintances, Professor Skeel, Sam Heller and Nick Johnson!
CHAPTER IX AT THE OLD MILL Difficult it would be to say which party was the more surprised. Certain it was, though Tom and his chums knew that their former teacher intended coming to the vicinity, and though they realized that Sam and Nick had gotten off the train with camping stuff near Wilden, they never expected to meet the three in this spot. And, for that matter, neither did Mr. Skeel and the two lads, with whom he seemed to be on friendly terms, think to behold Tom, for the former plainly showed the surprise he felt. “Well what do you know about this?” asked Jack, in a low voice. “It’s the limit,” agreed Tom. “Mind your wheel or you’ll have us on shore,” said Bert. “There’s a big rock just ahead of you.” Tom shifted the wheel with a rapid turn. He had been so interested in looking at the trio on shore that he had not noticed where he was steering. “Shall we speak to ’em?” asked Jack. “No, don’t,” advised Bert. “There’s no use getting into an argument.” “And yet we might find out something about them, and what they are doing up here,” insisted Jack, who generally liked to take the initiative. “I guess we’d better not,” spoke Tom. “Anyhow, they wouldn’t give us any satisfaction. If they hail us we’ll answer, and that’s all.”
But the three on shore evidently had no intention of speaking. After his first stare of surprise Mr. Skeel was seen to speak to Sam and Nick, and then, with a final glance at our friends, the trio turned and plunged back into the woods. “Well, that’s over—for the time being,” remarked Dick. “Yes,” assented Bert. “Can you see which way they’re going, Tom?” “Why should we want to?” “Because they may be going to the same place we are.” “What, to the old mill?” “Sure.” “They don’t know anything about it,” declared Tom. “How do you know? That story of buried treasure is more or less known all over this section, and the hunt old Wallace is making for it, too. So why shouldn’t Mr. Skeel, and Sam or Nick know of it?” “Well, maybe you’re right,” agreed Tom. “But we can’t see which way they’re headed. The brush is too thick.” “We’re not far from the mill, if I’m any judge,” said Jack. “Why?” Tom wanted to know. “How can you tell? You’ve never been there.” “No, but there’s a current setting into the lake now, and that means the river isn’t far off. The mill is on the river, so, naturally we’re near the mill. Q. E. D., as we used to say after we’d floundered through a geometry proposition.” “Well, maybe you’re right,” admitted our hero. “Another thing,” went on Jack. “If we’re near the mill, so are those fellows. So you see——” “By Jove!” cried Tom. “I shouldn’t be surprised but what you were right, Jack. This man Skeel would be up to any proposition to make
money, and he may, as you say, have heard the rumor of treasure in the old mill.” “How do you account for him meeting Sam and Nick?” asked Bert. “Oh, it probably just happened,” suggested Tom. “If they are camping near here, and Skeel is doing the same thing, it’s not out of reason that they should meet. Well, if they’re after the treasure in the old mill I don’t see what’s to prevent us having a go for the same thing.” “If Old Wallace will let us,” put in Bert. “Oh, well, we’ll have to take a chance with him,” said Tom. “We’ll have to wait until he’s away from home, which he seems to be most of the time.” “And if we get the treasure, what will we do with it?” inquired Dick. “Wait until we do,” laughed Tom. “I don’t believe there is one chance in a thousand of there being any treasure there, and if there is, it’s a hundred to one shot that we can’t find it, nor can anyone else. But it will be fun to have a go for it.” “And if we do find it,” put in Jack, “we’ll all take a trip to Europe.” “No,” spoke Tom, quietly, “if we do find any treasure, it will have to go to the one who owns it—the old hermit, very likely.” “Oh pshaw!” cried Jack. “After the mean way he treated us, Tom?” “Sure. Right is right. But say, don’t let’s get into an argument over such a remote possibility. Wait until we get to the mill, and have a look around. I’m an expert on buried treasure, and I can tell, as soon as I see a place, what the prospects are,” and Tom’s chums joined in his hearty laugh. “Well, speed up,” suggested Jack, “and we’ll see what sort of an Eldorado lies before us. Westward ho!” and he struck a dramatic attitude.
Tom turned on more gasolene and advanced the spark, so that the Tag shot ahead. There was no further sign of Professor Skeel and the two boys. “There’s the river!” exclaimed Bert, about a quarter of an hour later, as the boat went around a bend, and they came into view of a stream flowing into the lake. It was as wild and picturesque as the lake itself, with big trees on either bank, overhanging the water in places. “Say, that’s great!” cried Tom. “I’m going to get some pictures of that. Take the wheel, Jack, while I get out my camera.” Tom was soon snapping away, getting a number of fine views, while with Jack at the wheel, and Dick to watch the motor, the Tag swept slowly into the river. The current was not strong at this point, and it was possible to slow down to half speed, as the lads did not know the character of the water, nor how much depth there was, though the Tag did not draw more than two feet. “Let’s see who’ll spot the old mill first,” proposed Tom, as he adjusted his camera to take more pictures when the ruin should be sighted. “I’d rather get the first sight of the hidden treasure,” declared Jack, who seemed to have more faith in the existence of the secret horde than did the others. “Anyone can see a mill,” he went on, “but it takes an eagle eye to spot treasure.” “And I suppose you think you’ve got the eagle eye!” laughed Bert. “Sure I have. Say, Dick, isn’t it almost lunch time?” “I don’t know. I’m not the cook this week. It’s up to Tom.” “Can we eat, Tom?” asked his roommate at Elmwood Hall. “Not until we get to the mill. Work before pleasure, my boy. That’s the rule here.” “Well then, get ready with the grub,” said Jack, quietly, “for there’s your mill,” and he pointed just ahead of them.
“By Jove! So it is!” exclaimed Tom. They had gone around a turn in the river, and on one bank, situated on a little rise, were the ruins of an old stone mill. In its day it had been a big structure, built of field stone, and it must have been a substantial place to which the settlers for miles around probably came with their grain. But now it was in ruins, through the ravages of time and the hands of those who sought the treasure. As the boat approached it the boys could see where a flume had been built to take the water from the river, and direct it over a big wheel. Of the latter there was little left. Trees and underbrush grew up close to the old structure, near which were the rotting remains of a wharf where, in the olden days, likely, the craft of the settlers had tied up when they came with grist. “Say, it’s a wonderful ruin all right,” said Tom in a low voice. “Put over to shore, Jack, while I get a picture. Then we’ll get out and have a look around.” As Tom focused his camera, and clicked the shutter, there was a movement in the tangle of vines and bushes near what had been the main entrance to the mill. “Look out!” exclaimed Jack. “Some one’s coming!”
CHAPTER X A CURIOUS CONFERENCE Holding themselves in readiness for whatever they might see, or for whatever might happen, the boys peered anxiously toward the place whence the noise and movement came. “False alarm!” laughed Tom, as a fox leaped into view and then, seeing human enemies, slunk out of sight. “It made noise enough for a man,” declared Jack. “I sure thought it was the hermit getting ready to repel boarders.” “And treasure seekers,” added Dick. “Well, let’s go ashore,” suggested Bert. “That is, if Tom is done taking fancy snap shots of the old ruin.” “Sure, I’ve got pictures enough for now, though I may want some from the other side,” assented our hero. Making the boat fast to the rotting wharf, the four lads climbed out and made ready to inspect the old ruin. “Look out!” suddenly called Tom. “That’s a weak plank you’re stepping on, Jack. You’ll be through it in another minute!” He made a grab for his chum, but it was too late. Jack, who had hurried on in advance of the others, had stepped on a board of the wharf that was but a mere rotten shell, and, an instant later, one foot went through it, and Jack slipped down to his hip, the other leg doubled up under him. “Help! Help!” he cried, in mock seriousness. “One foot’s in the water, and the other will be in a minute.”
“Are you hurt?” asked Bert anxiously. “No, but if this leg isn’t skinned all the way up I’m a loon. Pull me out, can’t you?” As Bert and Dick started toward him Tom called: “Stand back! If we all crowd up on those old boards we will all be through. Wait until I can lay another plank down, that isn’t so near gone. Then we can give you a hand.” With the aid of Bert and Dick, our hero ripped off a more substantial board, and then, stepping on this they managed to pull Jack from his uncomfortable position, for he could not help himself. “Well, how about you?” asked Tom, when they had all made their way off the old wharf to shore. “Oh, so-so. I’m badly battered up, but still in the ring. One foot is well soaked, but it’s warm weather and I guess I won’t get the epizootic. Say, though, I’m going to be lame,” and Jack limped along. An examination showed that his right leg was painfully skinned and bruised, where it had scraped on the edges of the hole in the plank, as his foot went through the timber. “We’ll bandage it up when we get to camp,” said Tom, as he used an extra handkerchief on the worst cut of his chum’s leg. “Do you feel able to go on to the mill, or shall we turn back, Jack?” “Go on, of course,” declared the injured one. “I’m not going to let a little thing like a game leg stand between me and a treasure hunt. Lead on, captain!” “That’s the talk!” exclaimed Bert. “You’ll get the best of the pirates’ hoard yet.” “Now go a bit easy,” cautioned Tom. “It may be that Old Wallace is around somewhere, and, as this is his property, he’d be justified in making a row if he found us here. So go a bit slow until we size up the situation.”
They were on the lower side of the mill now, the side nearest the river. The ancient structure consisted of three stories. The lower one was a sort of basement, on a level with the lower ground, where it was evident that wagons had driven in to receive their loads of grain. Here too, was some of the old machinery of the mill, the levers that controlled the water gate and other things, but now all rotted and fallen into decay. “Say, this would be the place where the treasure would be buried, if anywhere,” declared Jack. “I don’t think so,” spoke Tom. “It’s too conspicuous.” “That’s just it,” argued Jack. “The more conspicuous a thing is, the harder it is to find it, sometimes. Nothing is more difficult to pick up, sometimes, than something right under your nose, as the saying is.” “That’s right,” agreed Bert. “Did you ever play the geography game?” “No. What is it?” asked Tom. “Well, you take a big map, and ask a person to find some country, city, lake or river, as the case is. Most persons pick out for the puzzle a name printed in very small type, but those who know select a name printed in big letters, that take up half the map, maybe. And it most always happens that this is the hardest to find. I didn’t originate that,” he added, modestly. “I think Poe speaks of it in one of his stories.” “That’s right,” agreed Tom. “At any rate some one has had a try for the treasure here, at any rate, if signs of digging go for anything.” This was indeed so, for the ground was torn up, and in many places stones had been knocked out of the thick walls, as if some one had looked for secret hiding places. “Well, we can’t stop to dig now,” said Tom. “But if things go right we may later. Let’s go up on the main floor,” and he started toward an ancient doorway.
“Not there!” cried Jack, holding back his chum. “Why not?” “The boards there will be as rotten as those on the wharf, and we’ll all take a tumble. Let’s go outside and around on the solid earth. I don’t want to put my other leg out of commission,” and he limped out of the basement of the ancient mill. The others followed, and soon they stood in the doorway of what had evidently been the main entrance to the ancient structure. It was on a level with the higher ground, farther back from the river. This floor contained the mill-stones, now fallen from their position, and encumbered with wreckage. There were several rooms, opening one into the other, now that the doors had fallen from their hinges, and here were holes that went through to the floor above. These holes had once contained the chutes through which the grain was fed to the mill-stones. “There might be treasure here almost anywhere,” remarked Jack, as he looked about. “And it’s been pretty well grubbed for,” commented Tom. “They’ve almost ripped the insides out of the mill looking for it. I suppose old Wallace has cut and sawed and pulled apart until it’s a wonder the old mill hangs together.” “It’s a well-built old place,” said Dick. “The stone walls are thick. There may be a hiding place in them.” “I shouldn’t wonder,” and Bert shrugged his shoulders. “Well, it’s going to be a job to take them apart all right,” and he looked at the stones imbedded in mortar that was as good still as it was the day it was mixed. The boys wandered about the main floor, and looked for a place to ascend to the third story, but there seemed to be none. “If we had a rope we could make it,” said Tom. “We’ll bring one next time.”
“Huh! How you going to get up there to fasten it?” asked Bert. “Tie a stick on the end, throw the stick up, and when it catches, crossways, in one of the chute-holes we can go up easily enough.” “Good boy! Bright idea!” complimented Jack. “Well, let’s see if we can find where old Wallace hangs out. We haven’t come across his living quarters yet.” There were several rooms they had not yet explored, and they now proceeded to visit them. They had evidently been the living apartments of the former miller, but now they were pretty much in ruins. “No signs of a course dinner having been prepared here,” commented Tom. “It smells as musty as time. He must hang out somewhere else.” “Upstairs, I’ll wager,” said Dick. “But how does he get up?” asked Jack. “Oh, he has some secret way,” declared Tom. “We’ll have to get a rope and explore that third story all right.” “Say, maybe we’re staying too long now,” suggested Bert. “Old Wallace may come along and nab us. We’ve seen all there is to, I guess, except upstairs.” “But we haven’t seen any gold,” said Jack. “I want to find some before I go back.” “Get out!” laughed Tom. “All the gold there is in this mill you can put in your eye. But I think it might be a good idea to look outside a bit. Maybe there’s some outbuilding, or some secret cache where the pirates or settlers hid their stuff. We’ll take a look.” “And then we’ll have some eats!” suggested Jack. “I’m as hungry as the proverbial bear.” They strolled about the old mill, and saw more signs of where a search had been made for the reputed treasure. Holes innumerable
were on every side, but the attempts to locate the hidden gold had soon been given over, for the excavations were shallow. “Now for the eats!” exclaimed Jack, as they started for the dock where their boat was tied. “Lands! but I’m stiff!” He really was limping painfully, and his chums had to help him down the hill to the river. As they approached their boat Tom, who was slightly in advance, uttered an exclamation of surprise as he peered along a path that led up the river. “What is it?” asked Dick. “Look,” was the answer. “Old Wallace! We got away just in time.” “And see who’s with him!” exclaimed Jack, in a hoarse whisper. “Professor Skeel!” “By Jove! So it is!” gasped Tom. “Wonders will never cease. Have they seen us?” It was evident they had not, but to make sure of it the boys hurried behind a screen of bushes, where they could see but not be observed. “Look!” exclaimed Tom again. “They’re going to have a conference.” As he spoke the others could see that the former professor and the old hermit had come to a halt in a place where the path widened. It was in a little glade, and, sitting down beneath the trees, the two men, one of whom had played such a strange part in Tom’s life, and the other, who was destined to, proceeded to talk earnestly. What they said could not be heard, but it was evident that it was some subject that interested them both, for they held their heads close together as if afraid of being overheard. They little realized that they were being watched. “What are they doing now?” asked Dick. “The old hermit has some sort of a paper,” said Tom.
“And he’s showing it to Mr. Skeel,” added Bert. “Maybe it’s some sort of map to tell where the treasure is,” suggested Dick. “But why would he be showing it to our old professor?” asked Tom. “If he wants to keep it a secret why is he giving it away like that?” “Hard to say,” commented Jack. “I think, though——” He did not finish, for at that moment Mr. Skeel and the hermit leaped to their feet and gazed down the path as though they heard some one coming.
CHAPTER XI AN ANGRY HERMIT “Something new on the programme,” commented Tom in low voice. “Do you think they heard us?” asked Dick. “No, they couldn’t. And they don’t see us. They’re looking the other way,” said Jack. “But there’s something doing,” declared Bert. “I wonder what it is?” They had their answer a moment later, when there came into view around the bend in the path Sam Heller and Nick Johnson. “Well, I’ll be jiggered!” gasped Tom. “We meet them at every turn.” “I wonder how they got here so soon after we met them?” asked Bert. “It’s quite a distance to walk.” “Maybe they took a short cut,” suggested Jack. “Hush! Look what’s going on now,” advised Tom. As they glanced toward where the professor and the hermit had held a conference, they saw the old man transported into one of his fits of rage. He stamped about, and shook his fist at Sam and Nick, occasionally changing by making threatening gestures at Mr. Skeel. “Say, he’s the limit!” murmured Dick. “Listen,” cautioned Tom. “He’s saying something.”
“Leave here! Leave here at once!” commanded old Wallace, almost hitting the two lads as he shook his fists at them. “How dare you come on my property? You are after the treasure; are you? Well, you shall never find it! I will locate it! I will make the old mill give up its secret! Be off!” “Wait, wait,” said Mr. Skeel in a calm voice, laying his hand on the hermit’s arm. “Ha! You too are in a plot against me, I believe!” cried the angry hermit. “I am sorry I ever had anything to do with you. Go away!” and he took hold of the professor, and began shoving him away down the path. “One minute,” said Mr. Skeel in soothing tones, much different from the harsh ones he had almost constantly used in his classes at Elmwood Hall. “What is it you object to?” “These lads—what are they doing here? Are they spying on me?” and the aged man pointed at Nick and Sam. “They are my assistants,” said the professor soothingly, and, though he spoke in a low tone, Tom and his chums could hear him. “Without their aid I can not help you,” Mr. Skeel went on, and when the hermit’s back was turned toward him our hidden friends distinctly saw the professor make a signal of caution and of acquiescence toward the two lads, who craftily nodded their understanding. “Your assistants?” asked the hermit. “Yes. If you want me to help you I must have them to help me. I would have told you about them, but I did not get the chance until they came so unexpectedly. Had they known that you objected to their presence they would have remained away. But I assure you that you can trust them.” “Well,” said the hermit, bitterly, “since I have told you part of my secret, and trusted you with it, I suppose your assistants must be in on it. But no more! No more!” and he shook his fist toward the
clouds, and glanced around as though he feared more intruders. “There were some other boys around the other day,” the aged man went on, “and if I find them sneaking about my mill it will be the worse for them.” “Say, we did get away just in time,” whispered Jack. “That’s right,” agreed Dick. “But what in the world does Skeel mean by saying he is going to help Wallace, and that Sam and Nick are his assistants, I wonder?” asked Bert. “That’s easy to guess,” answered Tom. “Skeel, somehow or other, has heard about the treasure. Now he’s trying to soft-soap the hermit into letting him have a hunt for it. Probably he’s promised to turn most of it over to the old man.” “I think I see him doing it, if he finds it,” commented Bert. “And Skeel has the nerve to say that Sam and Nick are his helpers,” said Jack. “Hot helpers they are!” “Oh, that was just a bit of jollying, thought up on the spur of the moment,” declared Tom. “He didn’t figure on Sam and Nick following him, and he had to concoct some story to account for their presence. Though I don’t doubt but what Skeel, and those two cronies, are in thick about some scheme.” “Searching for the treasure?” asked Dick. “I believe so. Well, they’ve got one advantage of us, but maybe we can get ahead of them yet,” spoke Tom. “If only we can get a chance to do some exploring we’ll do it. But we can’t do anything more now.” “No, let’s go down to the boat and eat,” suggested Jack. “I’m still hungry.” “Wait a minute,” advised Tom. “I think they’re going to move on, and we don’t want to run into them.”
As they watched they saw Sam and Nick turn and retrace their steps back along the path. They had held a little conversation with Mr. Skeel, to one side, so that the hermit had not heard, though he eyed them suspiciously. Then Mr. Skeel and the old man resumed their talk. “Lucky that Sam and Nick didn’t come this way,” said Tom, as he helped Jack to stand up. “Now don’t make any more noise than you can help, or they may hear us.” “They’ll hear the boat when it starts,” said Dick. “I’ll drift down the river a bit before I crank up,” spoke Tom. “Come on, everybody.” They started down the bank, toward their boat, having come to a halt a little distance from it. Suddenly Dick, who was in the rear, uttered an exclamation. “What is it?” called Tom sharply. “They’re going away—Skeel and the hermit, and one of them has dropped a piece of paper on the path.” “A piece of paper!” exclaimed Tom. “We must have that! Here, wait a minute! If they don’t miss it, and come back for it, I’ll get it.” He crawled cautiously back to his former post of observation behind the screen of bushes.
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  • 5. MICROEMULSIONS Properties and Applications 89595_C000.indd i 89595_C000.indd i 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
  • 6. DANIEL BLANKSCHTEIN Department of Chemical Engineering Massachusetts Institute of Technology Cambridge, Massachusetts S. KARABORNI Shell International Petroleum Company Limited London, England LISA B. QUENCER The Dow Chemical Company Midland, Michigan JOHN F. SCAMEHORN Institute for Applied Surfactant Research University of Oklahoma Norman, Oklahoma P. SOMASUNDARAN Henry Krumb School of Mines Columbia University New York, New York ERIC W. KALER Department of Chemical Engineering University of Delaware Newark, Delaware CLARENCE MILLER Department of Chemical Engineering Rice University Houston, Texas DON RUBINGH The Procter & Gamble Company Cincinnati, Ohio BEREND SMIT Shell International Oil Products B.V. Amsterdam, The Netherlands JOHN TEXTER Strider Research Corporation Rochester, New York SURFACTANT SCIENCE SERIES FOUNDING EDITOR MARTIN J. SCHICK 1918–1998 SERIES EDITOR ARTHUR T. HUBBARD Santa Barbara Science Project Santa Barbara, California ADVISORY BOARD 89595_C000.indd ii 89595_C000.indd ii 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
  • 7. 1. Nonionic Surfactants, edited by Martin J. Schick (see also Volumes 19, 23, and 60) 2. Solvent Properties of Surfactant Solutions, edited by Kozo Shinoda (see Volume 55) 3. Surfactant Biodegradation, R. D. Swisher (see Volume 18) 4. Cationic Surfactants, edited by Eric Jungermann (see also Volumes 34, 37, and 53) 5. Detergency: Theory and Test Methods (in three parts), edited by W. G. Cutler and R. C. Davis (see also Volume 20) 6. Emulsions and Emulsion Technology (in three parts), edited by Kenneth J. Lissant 7. Anionic Surfactants (in two parts), edited by Warner M. Linfield (see Volume 56) 8. Anionic Surfactants: Chemical Analysis, edited by John Cross 9. Stabilization of Colloidal Dispersions by Polymer Adsorption, Tatsuo Sato and Richard Ruch 10. Anionic Surfactants: Biochemistry, Toxicology, Dermatology, edited by Christian Gloxhuber (see Volume 43) 11. Anionic Surfactants: Physical Chemistry of Surfactant Action, edited by E. H. Lucassen-Reynders 12. Amphoteric Surfactants, edited by B. R. Bluestein and Clifford L. Hilton (see Volume 59) 13. Demulsification: Industrial Applications, Kenneth J. Lissant 14. Surfactants in Textile Processing, Arved Datyner 15. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, edited by Ayao Kitahara and Akira Watanabe 16. Surfactants in Cosmetics, edited by Martin M. Rieger (see Volume 68) 17. Interfacial Phenomena: Equilibrium and Dynamic Effects, Clarence A. Miller and P. Neogi 18. Surfactant Biodegradation: Second Edition, Revised and Expanded, R. D. Swisher 19. Nonionic Surfactants: Chemical Analysis, edited by John Cross 20. Detergency: Theory and Technology, edited by W. Gale Cutler and Erik Kissa 21. Interfacial Phenomena in Apolar Media, edited by Hans-Friedrich Eicke and Geoffrey D. Parfitt 22. Surfactant Solutions: New Methods of Investigation, edited by Raoul Zana 23. Nonionic Surfactants: Physical Chemistry, edited by Martin J. Schick 24. Microemulsion Systems, edited by Henri L. Rosano and Marc Clausse 25. Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C. C. Gray 26. Surfactants in Emerging Technologies, edited by Milton J. Rosen 27. Reagents in Mineral Technology, edited by P. Somasundaran and Brij M. Moudgil 28. Surfactants in Chemical/Process Engineering, edited by Darsh T. Wasan, Martin E. Ginn, and Dinesh O. Shah 29. Thin Liquid Films, edited by I. B. Ivanov 89595_C000.indd iii 89595_C000.indd iii 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
  • 8. 30. Microemulsions and Related Systems: Formulation, Solvency, and Physical Properties, edited by Maurice Bourrel and Robert S. Schechter 31. Crystallization and Polymorphism of Fats and Fatty Acids, edited by Nissim Garti and Kiyotaka Sato 32. Interfacial Phenomena in Coal Technology, edited by Gregory D. Botsaris and Yuli M. Glazman 33. Surfactant-Based Separation Processes, edited by John F. Scamehorn and Jeffrey H. Harwell 34. Cationic Surfactants: Organic Chemistry, edited by James M. Richmond 35. Alkylene Oxides and Their Polymers, F. E. Bailey, Jr., and Joseph V. Koleske 36. Interfacial Phenomena in Petroleum Recovery, edited by Norman R. Morrow 37. Cationic Surfactants: Physical Chemistry, edited by Donn N. Rubingh and Paul M. Holland 38. Kinetics and Catalysis in Microheterogeneous Systems, edited by M. Grätzel and K. Kalyanasundaram 39. Interfacial Phenomena in Biological Systems, edited by Max Bender 40. Analysis of Surfactants, Thomas M. Schmitt (see Volume 96) 41. Light Scattering by Liquid Surfaces and Complementary Techniques, edited by Dominique Langevin 42. Polymeric Surfactants, Irja Piirma 43. Anionic Surfactants: Biochemistry, Toxicology, Dermatology, Second Edition, Revised and Expanded, edited by Christian Gloxhuber and Klaus Künstler 44. Organized Solutions: Surfactants in Science and Technology, edited by Stig E. Friberg and Björn Lindman 45. Defoaming: Theory and Industrial Applications, edited by P. R. Garrett 46. Mixed Surfactant Systems, edited by Keizo Ogino and Masahiko Abe 47. Coagulation and Flocculation: Theory and Applications, edited by Bohuslav Dobiás 48. Biosurfactants: Production Properties Applications, edited by Naim Kosaric 49. Wettability, edited by John C. Berg 50. Fluorinated Surfactants: Synthesis Properties Applications, Erik Kissa 51. Surface and Colloid Chemistry in Advanced Ceramics Processing, edited by Robert J. Pugh and Lennart Bergström 52. Technological Applications of Dispersions, edited by Robert B. McKay 53. Cationic Surfactants: Analytical and Biological Evaluation, edited by John Cross and Edward J. Singer 54. Surfactants in Agrochemicals, Tharwat F. Tadros 55. Solubilization in Surfactant Aggregates, edited by Sherril D. Christian and John F. Scamehorn 56. Anionic Surfactants: Organic Chemistry, edited by Helmut W. Stache 57. Foams: Theory, Measurements, and Applications, edited by Robert K. Prud’homme and Saad A. Khan 58. The Preparation of Dispersions in Liquids, H. N. Stein 59. Amphoteric Surfactants: Second Edition, edited by Eric G. Lomax 89595_C000.indd iv 89595_C000.indd iv 11/10/2008 10:02:46 PM 11/10/2008 10:02:46 PM
  • 9. 60. Nonionic Surfactants: Polyoxyalkylene Block Copolymers, edited by Vaughn M. Nace 61. Emulsions and Emulsion Stability, edited by Johan Sjöblom 62. Vesicles, edited by Morton Rosoff 63. Applied Surface Thermodynamics, edited by A. W. Neumann and Jan K. Spelt 64. Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal 65. Detergents in the Environment, edited by Milan Johann Schwuger 66. Industrial Applications of Microemulsions, edited by Conxita Solans and Hironobu Kunieda 67. Liquid Detergents, edited by Kuo-Yann Lai 68. Surfactants in Cosmetics: Second Edition, Revised and Expanded, edited by Martin M. Rieger and Linda D. Rhein 69. Enzymes in Detergency, edited by Jan H. van Ee, Onno Misset, and Erik J. Baas 70. Structure-Performance Relationships in Surfactants, edited by Kunio Esumi and Minoru Ueno 71. Powdered Detergents, edited by Michael S. Showell 72. Nonionic Surfactants: Organic Chemistry, edited by Nico M. van Os 73. Anionic Surfactants: Analytical Chemistry, Second Edition, Revised and Expanded, edited by John Cross 74. Novel Surfactants: Preparation, Applications, and Biodegradability, edited by Krister Holmberg 75. Biopolymers at Interfaces, edited by Martin Malmsten 76. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications, Second Edition, Revised and Expanded, edited by Hiroyuki Ohshima and Kunio Furusawa 77. Polymer-Surfactant Systems, edited by Jan C. T. Kwak 78. Surfaces of Nanoparticles and Porous Materials, edited by James A. Schwarz and Cristian I. Contescu 79. Surface Chemistry and Electrochemistry of Membranes, edited by Torben Smith Sørensen 80. Interfacial Phenomena in Chromatography, edited by Emile Pefferkorn 81. Solid–Liquid Dispersions, Bohuslav Dobiás, Xueping Qiu, and Wolfgang von Rybinski 82. Handbook of Detergents, editor in chief: Uri Zoller Part A: Properties, edited by Guy Broze 83. Modern Characterization Methods of Surfactant Systems, edited by Bernard P. Binks 84. Dispersions: Characterization, Testing, and Measurement, Erik Kissa 85. Interfacial Forces and Fields: Theory and Applications, edited by Jyh-Ping Hsu 86. Silicone Surfactants, edited by Randal M. Hill 87. Surface Characterization Methods: Principles, Techniques, and Applications, edited by Andrew J. Milling 88. Interfacial Dynamics, edited by Nikola Kallay 89. Computational Methods in Surface and Colloid Science, edited by Malgorzata Borówko 89595_C000.indd v 89595_C000.indd v 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 10. 90. Adsorption on Silica Surfaces, edited by Eugène Papirer 91. Nonionic Surfactants: Alkyl Polyglucosides, edited by Dieter Balzer and Harald Lüders 92. Fine Particles: Synthesis, Characterization, and Mechanisms of Growth, edited by Tadao Sugimoto 93. Thermal Behavior of Dispersed Systems, edited by Nissim Garti 94. Surface Characteristics of Fibers and Textiles, edited by Christopher M. Pastore and Paul Kiekens 95. Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications, edited by Alexander G. Volkov 96. Analysis of Surfactants: Second Edition, Revised and Expanded, Thomas M. Schmitt 97. Fluorinated Surfactants and Repellents: Second Edition, Revised and Expanded, Erik Kissa 98. Detergency of Specialty Surfactants, edited by Floyd E. Friedli 99. Physical Chemistry of Polyelectrolytes, edited by Tsetska Radeva 100. Reactions and Synthesis in Surfactant Systems, edited by John Texter 101. Protein-Based Surfactants: Synthesis, Physicochemical Properties, and Applications, edited by Ifendu A. Nnanna and Jiding Xia 102. Chemical Properties of Material Surfaces, Marek Kosmulski 103. Oxide Surfaces, edited by James A. Wingrave 104. Polymers in Particulate Systems: Properties and Applications, edited by Vincent A. Hackley, P. Somasundaran, and Jennifer A. Lewis 105. Colloid and Surface Properties of Clays and Related Minerals, Rossman F. Giese and Carel J. van Oss 106. Interfacial Electrokinetics and Electrophoresis, edited by Ángel V. Delgado 107. Adsorption: Theory, Modeling, and Analysis, edited by József Tóth 108. Interfacial Applications in Environmental Engineering, edited by Mark A. Keane 109. Adsorption and Aggregation of Surfactants in Solution, edited by K. L. Mittal and Dinesh O. Shah 110. Biopolymers at Interfaces: Second Edition, Revised and Expanded, edited by Martin Malmsten 111. Biomolecular Films: Design, Function, and Applications, edited by James F. Rusling 112. Structure–Performance Relationships in Surfactants: Second Edition, Revised and Expanded, edited by Kunio Esumi and Minoru Ueno 113. Liquid Interfacial Systems: Oscillations and Instability, Rudolph V. Birikh, Vladimir A. Briskman, Manuel G. Velarde, and Jean-Claude Legros 114. Novel Surfactants: Preparation, Applications, and Biodegradability: Second Edition, Revised and Expanded, edited by Krister Holmberg 115. Colloidal Polymers: Synthesis and Characterization, edited by Abdelhamid Elaissari 116. Colloidal Biomolecules, Biomaterials, and Biomedical Applications, edited by Abdelhamid Elaissari 117. Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications, edited by Raoul Zana and Jiding Xia 89595_C000.indd vi 89595_C000.indd vi 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 11. 118. Colloidal Science of Flotation, Anh V. Nguyen and Hans Joachim Schulze 119. Surface and Interfacial Tension: Measurement, Theory, and Applications, edited by Stanley Hartland 120. Microporous Media: Synthesis, Properties, and Modeling, Freddy Romm 121. Handbook of Detergents, editor in chief: Uri Zoller, Part B: Environmental Impact, edited by Uri Zoller 122. Luminous Chemical Vapor Deposition and Interface Engineering, HirotsuguYasuda 123. Handbook of Detergents, editor in chief: Uri Zoller, Part C: Analysis, edited by Heinrich Waldhoff and Rüdiger Spilker 124. Mixed Surfactant Systems: Second Edition, Revised and Expanded, edited by Masahiko Abe and John F. Scamehorn 125. Dynamics of Surfactant Self-Assemblies: Micelles, Microemulsions, Vesicles and Lyotropic Phases, edited by Raoul Zana 126. Coagulation and Flocculation: Second Edition, edited by Hansjoachim Stechemesser and Bohulav Dobiás 127. Bicontinuous Liquid Crystals, edited by Matthew L. Lynch and Patrick T. Spicer 128. Handbook of Detergents, editor in chief: Uri Zoller, Part D: Formulation, edited by Michael S. Showell 129. Liquid Detergents: Second Edition, edited by Kuo-Yann Lai 130. Finely Dispersed Particles: Micro-, Nano-, and Atto-Engineering, edited by Aleksandar M. Spasic and Jyh-Ping Hsu 131. Colloidal Silica: Fundamentals and Applications, edited by Horacio E. Bergna and William O. Roberts 132. Emulsions and Emulsion Stability, Second Edition, edited by Johan Sjöblom 133. Micellar Catalysis, Mohammad Niyaz Khan 134. Molecular and Colloidal Electro-Optics, Stoyl P. Stoylov and Maria V. Stoimenova 135. Surfactants in Personal Care Products and Decorative Cosmetics, Third Edition, edited by Linda D. Rhein, Mitchell Schlossman, Anthony O'Lenick, and P. Somasundaran 136. Rheology of Particulate Dispersions and Composites, Rajinder Pal 137. Powders and Fibers: Interfacial Science and Applications, edited by Michel Nardin and Eugène Papirer 138. Wetting and Spreading Dynamics, edited by Victor Starov, Manuel G. Velarde, and Clayton Radke 139. Interfacial Phenomena: Equilibrium and Dynamic Effects, Second Edition, edited by Clarence A. Miller and P. Neogi 140. Giant Micelles: Properties and Applications, edited by Raoul Zana and Eric W. Kaler 141. Handbook of Detergents, editor in chief: Uri Zoller, Part E: Applications, edited by Uri Zoller 142. Handbook of Detergents, editor in chief: Uri Zoller, Part F: Production, edited by Uri Zoller and co-edited by Paul Sosis 143. Sugar-Based Surfactants: Fundamentals and Applications, edited by Cristóbal Carnero Ruiz 144. Microemulsions: Properties and Applications, edited by Monzer Fanun 89595_C000.indd vii 89595_C000.indd vii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 12. 89595_C000.indd viii 89595_C000.indd viii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 13. MICROEMULSIONS Edited by Monzer Fanun Al-Quds University East Jerusalem, Palestine CRC Press is an imprint of the Taylor & Francis Group, an informa business Boca Raton London New York Properties and Applications 89595_C000.indd ix 89595_C000.indd ix 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 14. CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-8959-2 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, trans- mitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright. com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Fanun, Monzer. Microemulsions : properties and applications / Monzer Fanun. p. cm. -- (Surfactant science ; 144) ISBN 978-1-4200-8959-2 (alk. paper) 1. Emulsions. I. Title. TP156.E6F36 2008 660’.294514--dc22 2008029538 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com 89595_C000.indd x 89595_C000.indd x 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 15. Contents Foreword ............................................................................................................. xv Preface................................................................................................................xxi Editor ................................................................................................................xxv Contributors ....................................................................................................xxvii Chapter 1 A Phase Diagram Approach to Microemulsions.............................1 Stig E. Friberg and Patricia A. Aikens Chapter 2 Physicochemistry of W/O Microemulsions: Formation, Stability, and Droplet Clustering................................................... 17 Animesh Kumar Rakshit and Satya Priya Moulik Chapter 3 Percolating Phenomenon in Microemulsions: Effect of External Entity ............................................................... 59 S. K. Mehta, Khushwinder Kaur, Gurpreet Kaur, and K. K. Bhasin Chapter 4 Influence of Polyethylene Glycols and Polyethylene Glycol Dimethyl Ethers upon the Internal Dynamics of Water in Oil Microemulsions................................................................... 77 A. Cid, L. García-Río, D. Gómez-Díaz, and J. C. Mejuto Chapter 5 Microemulsions with Mixed Nonionic Surfactants ...................... 87 Monzer Fanun Chapter 6 Simple Alcohols and Their Role in the Structure and Interactions of Microemulsion Systems ................................143 Matija Tomšič and Andrej Jamnik Chapter 7 Formation and Characterization of Emulsified Microemulsions........................................................................... 185 Anan Yaghmur, Liliana de Campo, and Otto Glatter xi 89595_C000.indd xi 89595_C000.indd xi 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 16. xii Contents Chapter 8 Dynamics of Solvent and Rotational Relaxation of RTILs in RTILs-Containing Microemulsions........................................ 203 Debabrata Seth and Nilmoni Sarkar Chapter 9 Microemulsion Systems and Their Potential as Drug Carriers.......................................................................... 247 Raid G. Alany, Gamal M. M. El Maghraby, Karen Krauel-Goellner, and Anja Graf Chapter 10 Physicochemical Characterization of Pharmaceutically Applicable Microemulsions: Tween 40/Imwitor 308/ Isopropyl Myristate/Water .......................................................... 293 Mirjana Gašperlin and Marija Bešter-Rogač Chapter 11 Places of Microemulsion and Emulsion in Cancer Therapy: In Vitro and In Vivo Evaluation...................................................313 Ercüment Karasulu, Burçak Karaca, Levent Alparslan, and H. Yesim Karasulu Chapter 12 Enzyme Kinetics as a Useful Probe for Micelle and Microemulsion Structure and Dynamics ..............................331 Werner Kunz, Didier Touraud, and Pierre Bauduin Chapter 13 Biocatalysis in Microemulsions .................................................. 349 A. Xenakis, V. Papadimitriou, H. Stamatis, and F. N. Kolisis Chapter 14 Microemulsions as Decontamination Media for Chemical Weapons and Toxic Industrial Chemicals................................... 387 Thomas Hellweg, Stefan Wellert, Hans-Juergen Altmann, and André Richardt Chapter 15 Microemulsions as Potential Interfacial Chemical Systems Applied in the Petroleum Industry...............................................411 Afonso Avelino Dantas Neto, Tereza Neuma de Castro Dantas, Maria Carlenise Paiva de Alencar Moura, Eduardo Lins de Barros Neto, and Alexandre Gurgel 89595_C000.indd xii 89595_C000.indd xii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 17. Contents xiii Chapter 16 Nanoparticle Formation in Microemulsions: Mechanism and Monte Carlo Simulations...................................................... 451 M. de Dios, F. Barroso, and C. Tojo Chapter 17 Nanoparticle Uptake by (W/O) Microemulsions.........................465 Maen M. Husein and Nashaat N. Nassar Chapter 18 TiO2 Nanoparticles in Microemulsion: Photophysical Properties and Interfacial Electron Transfer Dynamics.............. 483 Hirendra N. Ghosh Chapter 19 Microemulsions as Pseudostationary Phases in Electrokinetic Chromatography: I. Estimation of Physicochemical Parameters. II. Analysis of Drugs in Pharmaceutical and Biofluidic Matrices ............................................................... 501 Valeria Tripodi and Silvia Lucangioli Index................................................................................................................ 527 89595_C000.indd xiii 89595_C000.indd xiii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 18. 89595_C000.indd xiv 89595_C000.indd xiv 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 19. Foreword MICROEMULSIONS—A VITAL FUNDAMENTAL RESEARCH AREA MOVING RAPIDLY INTO APPLICATIONS WHILE HAVING ITS SCIENTIFIC BASIS IN OTHER SURFACTANT SELF-ASSEMBLY SYSTEMS This book focuses on the properties and applications of microemulsions and, in particular, on their interrelationship. Of late, microemulsions have become a pop- ular subject and applications are emerging rapidly; this further stimulates the fun- damental studies. Therefore, this book is very timely and I congratulate the editor, Professor Monzer Fanun, for having prepared a volume with this focus and, in particular, achieving this so well by assembling an impressive list of contributors; this list is a good mix of established leading scientists and young colleagues enter- ing the field recently as they will be the ones who will continue to develop our research area. The history of microemulsions has been full of ups and downs and has been involved in many heated controversies. The name “microemulsions” itself has no doubt contributed strongly to confusion. Microemulsions are not micro but nano and are not emulsions. The history of microemulsion research is complex [1] and needs to be recounted since it provides important lessons. Here I rather wish to make a few comments from my experience with the evolution of microemulsion, which have a direct bearing on this book and its relevance. What is a microemulsion? This question was very much in focus when I first came in contact with the field by the end of the 1960s and early 1970s. The very fact that a question like this arises leads to considerable confusion and unneces- sary work. Thus, had the true nature of microemulsions been understood, a resort to the basic literature on surfactant self-assembly would have given logical expla- nations to many observations. Thirty years ago, when I had just been appointed to the chair of physical chemistry at Lund University, Professor Ingvar Danielsson from Åbo Akademi in Turku, Finland came for a sabbatical. Åbo Akademi was the world-famous institu- tion for physical chemistry where the founder of the institution, Per Ekwall, along with pupils such as Ingvar Danielsson, Krister Fontell, and Leo Mandell, had developed much of our fundamental understanding of surfactant systems, including micellization, phase behavior, and liquid crystallinity. On his retirement from Åbo, Ekwall moved to Stockholm to found the Laboratory (later Institute) of Surface Chemistry, while Danielsson took over his chair in Åbo. My first contacts with surfactant science and much of my learning were with this Stockholm-Åbo research community. xv 89595_C000.indd xv 89595_C000.indd xv 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 20. Having been concerned with aspects of surfactant aggregation on the macro- scopic and aggregate levels, Danielsson took interest in a deeper molecular level of understanding, using some novel nuclear magnetic resonance (NMR) approaches, which I had developed along with my colleagues in Lund. These studies included microemulsions and, discussing the research results and reading the literature, we became more and more concerned about the fact that different authors had very different opinions on microemulsions. (It is interesting that Ekwall and Fontell refused to use this term even though they were behind some of the pioneering and still central observations on microemulsions. Since the term referred to thermodynamically stable solutions, they found it a misno- mer.) Therefore, we found it timely to suggest a definition of microemulsion as the following: A microemulsion is a system of water, oil, and amphiphile, which is a single optically isotropic and thermodynamically stable liquid solution [2]. We also gave several examples of what we considered should be included in microemulsions and what should not. Looking into the contents of this book, and contemplating Monzer’s invita- tion to write this foreword, I found it of interest to examine a little the accep- tance of our definition among colleagues. While having a general impression that, after a quite long period of questioning, it became more and more accepted, I found it of interest to examine this further by a citation analysis. Our short note is certainly not a significant scientific contribution, but it is quite well cited (and is in fact among my 10 most cited papers). However, the citations show a very unusual variation over time, the distribution being pronouncedly “bimodal.” In the first years after publication, there is quite a constant modest citation frequency. Thereafter, there is a very pronounced peak in 1989, indicating that this is the year that a more general acceptance was obtained. Afterwards, citations decrease strongly and one would have expected that the paper would as usual start to become forgotten. However, a few years ago, citations started to increase in number again and, from the citations during the first half of 2008, we can guess that this year will give the largest number of citations so far. Why is that so? Some clue can be obtained from the field of the journals where the paper is cited. Thus in 1989, most citations were in journals that focused more on physical and colloid chemistry. The pattern is very different in 2008. A majority of the citations are in journals dealing with more applied aspects, in particular, in the pharmaceutical sciences. Is there any other evidence that microemulsions are now becoming better understood in the applied sciences, like pharmaceutics? An indication can be obtained by considering textbooks. A leading textbook in pharmacy is Physico- chemical Principles of Pharmacy by A. T. Florence and D. Attwood [3]. Both the placing of microemulsions in the book and the text devoted to this topic reveal that even in 1998, when the third edition was published, microemulsions had received very little attention in the pharmaceutical field and that, furthermore, the nature of microemulsions was misunderstood. Thus, while there were lengthy multipage descriptions of surfactant micellization, liquid crystallinity, vesicles, and solubi- lization, microemulsions were dealt with in a mere seven line paragraph starting xvi Foreword 89595_C000.indd xvi 89595_C000.indd xvi 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 21. Foreword xvii “Microemulsions, or so-called swollen micellar systems, consist of apparently homogeneous transparent systems of low viscosity which contain a high percent- age of both oil and water and high concentrations (15%–25%) of emulsifier mixture.” The misconception of microemulsions in the pharmaceutical field is accentuated by the fact that rather than being placed together with other thermo- dynamically stable surfactant self-assembly systems, it is considered as a type of dispersion and placed under the general heading “Emulsions, Suspensions, and Other Dispersions.” It is indicated from the citation analysis mentioned that if a corresponding textbook is prepared today, microemulsions would receive much more attention and would be properly classified and treated in conjunction with related surfactant systems, like micelles and liquid crystals, as they have indeed been in textbooks of physical chemistry and colloid chemistry for a long time. This book contains significant contributions regarding the applications of microemulsions for pharmaceutical formulations, as well as for other applications, and will no doubt help considerably to provide an excellent basis for applications into new fields. Regarding the long-standing issue of the confusion of treating microemulsions as one type of emulsion, Chapter 7 by Otto Glatter and coauthors, dealing with emulsified microemulsions, is particularly enlightening as it clearly hints to this misconception. Stig Friberg was certainly the pioneer who demonstrated that microemulsions are indeed thermodynamically stable solutions and, therefore, should be described by phase diagrams with respect to their stability. The significance of his work on the phase behavior of surfactant–oil–water systems for the development of the microemulsion field cannot be overestimated and it is indeed very appropriate that he was invited to write the first chapter of this book. I was myself very fortunate to have early contacts with Stig Friberg. In addition, I was strongly influenced and helped by the phase diagram work of two other pioneers in the field, Per Ekwall, already mentioned above, and Kozo Shinoda in Yokohama. Several of my collaborations with Friberg, Shinoda, and Ekwall concerned microemulsion microstructure, where they provided enlightening systems for structural investigation and deep insight into the subject. I consider my most important contribution to the field of microemulsion as being the first, together with coworkers, to demonstrate microemulsion bicontinu- ity. However, this work also nicely demonstrates how important it is in microemul- sionresearchtohaveabroaderperspective,inparticular considering other surfactant phases. My first study dealing with surfactant phase bicontinuity did not thus concern microemulsions but cubic liquid crystalline phases. In preparing a chapter dealing with applications of NMR for a book on Liquid Crystals and Plastic Crystals [4], I became confused when I came to the cubic phases.As we know, cubic phases can be located in different concentration ranges in a phase diagram, inter alia between the micellar solutions and the normal hexagonal phase, and between the hexagonal and the lamellar phases. I soon realized that the surfactant self-diffusion would be very different for discrete aggregates and for connected structures. This would 89595_C000.indd xvii 89595_C000.indd xvii 11/10/2008 10:02:47 PM 11/10/2008 10:02:47 PM
  • 22. thus be an interesting possibility for solving the problem of the structure of cubic liquid crystalline phases. A few experiments with a postdoctoral fellow, Tom Bull, at the new pulsed NMR spectrometer, giving differences in surfactant diffusion by orders of magnitude between the two cubic phases, could directly prove that one was built up of discrete micelles while the other was bicontinuous [5]. The cubic phase, which is more dilute in surfactant, was thus found to be characterized by very slow surfactant diffusion and thus must consist of (more or less stationary) discrete aggregates. In the more concentrated cubic phase, surfactant diffusion was found to be more than one order of magnitude faster. This rather surprising finding could only be understood if the surfactant molecules could diffuse freely over macroscopic distances; thus surfactant aggregates are connected. The distinction between discrete “droplet” structures and bicontinuous ones became central in the subsequent studies on microemulsions in Lund [6–12]. This research topic became even more emphasized when Peter Stilbs introduced the Fourier transform version of the NMR technique [13–16]. That surfactant self-assembly systems, which include liquid crystalline phases and isotropic solutions, can be divided into those that have discrete self-assembly aggregates and those where the aggregates are connected in one, two, or three dimensions was very clear for the pioneers of the microemulsion field mentioned above. Regarding lamellar phases, the two-dimensional connectivity was already appreciated at a very early stage. The same holds true for the (“normal” and “reverse”) hexagonal phases, although erroneous models of linearly associated spherical micelles, “pearls-on-a-string,” can be found in the literature; such a linear association was also, again incorrectly, advanced to explain droplet growth in microemulsions. The general acceptance of connectivity for these anisotropic phases stood in sharp contrast to a great difficulty to get an acceptance for bicontinu- ity for other phases. This is partly related to the fact that contrary to these anisotro- pic phases, it has been much more difficult to structurally characterize the different isotropic phases found in simple and complex surfactant systems: cubic liquid crystals, solutions in binary surfactant–water systems, and microemulsions. The first verification was due to observations of molecular self-diffusion over macro- scopic distances. Electrical conductivity offers a partial insight in providing infor- mation on the extension of aqueous domains. Fluorescence quenching can provide information on the growth of nonpolar domains, but a probe has to be introduced. Later cryogenic transmission electron microscopy has developed into a very impor- tant tool for imaging different surfactant phases. Using a similar approach as for cubic phases, it was thus quite straightforward to address the problem of microemulsion structure. Thus, by measuring oil and water self-diffusion, it was quite easy to establish whether oil or water or none of them are confined to discrete domains, “droplets.” In the first work on microemul- sion structure by self-diffusion, using both tracer techniques and NMR spin-echo measurements, it was clearly shown that, in addition to droplet microemulsions, over wide ranges of composition they can be bicontinuous [6]; this is manifested by both oil and water diffusion being rapid, not much less than the self-diffusion of the neat liquids. xviii Foreword 89595_C000.indd xviii 89595_C000.indd xviii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 23. Foreword xix The self-diffusion approach to microstructure is not limited to cubic phases or microemulsions. An early study concerned the demonstration of micellar growth into worm-like structures for nonionic surfactants [17,18]. Parallel pioneering studies on phase behavior of nonionic surfactants by Gordon Tiddy [19] also illus- trated the same feature. Another problem, soon to be tackled, was that of the microstructure of the “sponge phases,” a “microemulsion analogue,” for binary surfactant systems, termed L3 by Ekwall (and identified by him in a number of systems). While isotropic solutions in simple surfactant–water mixtures were for a long time considered synonymous with solutions of discrete surfactant micelles, there were indications of a more complex situation given by the clouding and phase separation into two solutions of nonionic surfactants at elevated temperature. Here self-diffusion was again expected to provide the solution [19,20]. For the sponge phase, water diffusion was much reduced compared to classical micellar solu- tions. In fact, it was close to 2/3 of the value of neat water. The surfactant diffusion was, on the other hand, found to be much more rapid, and close to 2/3 of the dif- fusion of the neat liquid surfactant, than what was observed for previously studied micellar solutions. The solutions are thus bicontinuous. These systems are perfect illustrations of bicontinuity and in many respects useful models of bicontinuous microemulsions. Both the water and surfactant self-diffusion coefficients are close to 2/3 of the values of the neat liquids, corresponding to an ideal zero mean curva- ture bicontinuous structure. While I have illustrated here, with some examples from our own research, how progress in our understanding has been dependent on understanding alterna- tive surfactant phases, this approach is certainly not unique. Several pioneers like Friberg, Ekwall, Shinoda, Tiddy, Scriven, and Wennerström, have provided beau- tiful examples of such a “holistic” view. It is my firm belief that in the ongoing expan- sion of the microemulsion field, that the present book emphasizes and supports a broader look into surfactant self-assembly and a resort to simpler surfactant sys- tems are mandatory. Björn Lindman Coimbra University and Lund University REFERENCES 1. Lindman, B.; Friberg, S. Microemulsions—a historical overview. In Handbook of Microemulsion Science and Technology, P. Kumar and K. L. Mittal, eds. Marcel Dekker, New York, 1999, pp. 1–12. 2. Danielsson, I.; Lindman, B. The definition of microemulsion. Colloids Surfaces 3, 1981, 391–392. 3. Florence, A.T; Attwood, D. Physicochemical Principles of Pharmacy, 3rd edn, Pharmaceutical Press, London, 1998. 4. Johansson, Å.; Lindman, B. In Liquid Crystals and Plastic Crystals, Nuclear Magnetic Resonance Spectroscopy of Liquid Crystals-Amphiphilic Systems, G.W. Gray and P. A. Winsor, eds. Ellis Horwood Publishers, Chichester, 1974, Vol. 2, pp. 192–230. 89595_C000.indd xix 89595_C000.indd xix 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 24. 5. Bull, T.; Lindman, B. Amphiphile diffusion in cubic lyotropic mesophases. Mol. Cryst. Liquid Cryst. 28, 1975, 155–160. 6. Lindman, B.; Kamenka, N.; Kathopoulis, T.M.; Brun, B.; Nilsson, P.G. Translational diffusion and solution structure of microemulsions. J. Phys. Chem. 84, 1980, 2485–2490. 7. Nilsson, P. G.; Lindman, B. Solution structure of nonionic surfactant microemul- sions from NMR self-diffusion studies. J. Phys. Chem. 86, 1982, 271–279. 8. Guéring, P.; Lindman, B. Droplet and bicontinuous structures in cosurfactant micro- emulsions from multi-component self-diffusion measurements. Langmuir 1, 1985, 464–468. 9. Olsson, U.; Shinoda, K.; Lindman, B. Change of the structure of microemulsions with the HLB of nonionic surfactant as revealed by NMR self-diffusion studies. J. Phys. Chem. 90, 1986, 4083–4088. 10. Lindman, B.; Shinoda, K.; Olsson, U.; Anderson, D.; Karlström, G.; Wennerström, H. On the demonstration of bicontinuous structures in microemulsions. Colloids Surfaces 38, 1989, 205–224. 11. Lindman, B.; Olsson, U. Structure of microemulsions studied by NMR Ber. Bunsen- ges. Phys. Chem. 100, 1996, 344–363. 12. Shinoda, K.; Lindman, B. Organized surfactant systems: Microemulsions. Lang- muir 3, 1987, 135–149. 13. Stilbs, P.; Moseley, M. E. Nuclear spin-echo experiments on standard Fourier-trans- form NMR spectrometers—Application to multi-component self-diffusion studies. Chem. Scripta 13, 1979, 26–28. 14. Stilbs, P. Prog. Fourier transform pulsed-gradient spin-echo studies of molecular diffusion. NMR Spectrosc. 19, 1987, 1–45. 15. Stilbs, P.; Moseley, M. E.; Lindman, B. Fourier transform NMR self-diffusion mea- surements on microemulsions. J. Magn. Reson. 40, 1980, 401–404. 16. Lindman, B.; Stilbs, P.; Moseley, M. E. Fourier transform NMR self-diffusion and microemulsion structure. J. Colloid Interface Sci. 83, 1981, 569–582. 17. Nilsson, P. G.; Wennerström, H.; Lindman, B. Structure of micellar solutions of nonionic surfactants. NMR self-diffusion and proton relaxation studies of poly(ethyleneoxide) alkylethers. J. Phys. Chem. 87, 1983, 1377–1385. 18. Lindman, B.; Wennerström, H. Nonionic micelles grow with increasing tempera- ture. J. Phys. Chem. 95, 1991, 6053–6054. 19. Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Phase behaviour of polyoxyethylene surfactants with water. Mesophase structures and partial miscibility (cloud points). Chem. Soc. Faraday Trans. 79, 1983, 975–1000. 20. Nilsson, P. G.; Lindman, B. Nuclear magnetic resonance self-diffusion and proton relaxation studies of nonionic surfactant solutions. Aggregate shape in isotropic solutions above the clouding temperature. J. Phys. Chem. 88, 1984, 4764–4769. 21. Lindman, B.; Olsson, U.; Stilbs, P.; Wennerström, H. Comment on the self-diffusion in L3 and other bicontinuous surfactant solutions. Langmuir 9, 1993, 625–626. xx Foreword 89595_C000.indd xx 89595_C000.indd xx 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 25. xxi Preface Microemulsions are microheterogeneous, thermodynamically stable, sponta- neously formed mixtures of oil and water under certain conditions by means of surfactants, with or without the aid of a cosurfactant. The first paper on microemulsions appeared in 1943 by Hoar et al., but it was Schulman and coworkers who first proposed the word “microemulsion” in 1959. Since then, the term “microemulsions” has been used to describe multicomponent systems comprising nonpolar, aqueous, surfactant, and cosurfactant components. The application areas of microemulsions have increased dramatically during the past decades. For example, the major industrial areas are fabricating nanopar- ticles, oil recovery, pollution control, and food and pharmaceutical industries. This book is a comprehensive reference that provides a complete and system- atic assessment of all topics affecting microemulsion performance, discussing the fundamental characteristics, theories, and applications of these dispersions that have been developed over the last decade. The book opens with a chapter that describes a phase diagram approach to microemulsions by two leading authorities (Friberg and Aikens) who have con- tributed significantly to the field of microemulsions. In the next three chapters, Moulik and Rakshit, Mehta and coworkers, and Mejuto and coworkers, respectively, advance different approaches to describe the percolation phenomenon in microe- mulsion systems. Theories that predict droplet clustering along with the basic conditions required for the formation and stability of these reverse micellar systems and the composition, temperature, and pressure-dependent conductance percolation and energetics of droplet clustering are reviewed. The influence of different additives on the conductance percolation of ionic microemulsions is also reviewed. Significant progress has been made in the formulation and characterization of new microemulsion systems. Properties of microemulsions with mixed non- ionic surfactants and different types of oils are reviewed in Chapter 5 by Fanun. A comprehensive review on the influence of various simple alcohols on the internal structural organization of microemulsion systems is presented in Chapter 6 by Tomšič and Jamnik. Chapter 7 by Glatter and coworkers focuses on the effect of variations in temperature and solubilizing oil on the formation and the reversible structural transitions of emulsified microemulsions that have excellent potential in applications such as nanoreactors or host systems for solubilizing active molecules in cosmetic, pharmaceutical, and food industries. The interaction of water with room temperature ionic liquids (RTILs) has been studied in RTIL/surfactant/water-containing ternary microemulsions by solvent and rotational relaxation of neutral Coumarin probes, namely Coumarin 153 and Coumarin 151, using steady-state and picosecond time-resolved emission spectroscopy, reviewed by Seth and Sarkar in Chapter 8. 89595_C000.indd xxi 89595_C000.indd xxi 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 26. xxii Preface Microemulsions accommodate poorly soluble drugs (both hydrophilic and lipophilic) and protect those that are vulnerable to chemical and enzymatic deg- radation. They have the potential to increase the solubility of poorly soluble drugs, enhance the bioavailability of drugs with poor permeability, reduce patient variability, and offer an alternative for controlled drug release. In Chap- ter 9, Alany and coworkers review the formulation and characterization of microemulsions intended for drug delivery applications. Recent investigations on pharmaceutically applicable microemulsions are described in Chapter 10 by Gašperlin and Bešter-Rogač. The use of emulsions and microemulsions as a delivery system for cancer therapy is described in Chapter 11 by Karasulu and coworkers. Enzymes when hosted in reverse micelles can catalyze reactions that are not favored in aqueous media. Products of high-added value can be thus pro- duced in these media. The potential technical and commercial applications of enzyme-containing microemulsions as microreactors are mainly linked to their unique physicochemical properties. The potential biotechnological applications of microemulsions with immobilized biocatalysts such as enzymes are described in Chapter 12 by Kunz and coworkers and in Chapter 13 by Xenakis and coworkers. Great efforts have been made in order to replace established but harmful, corrosive, and therefore, obsolete decontamination media for chemical warfare agents and toxic industrial chemicals. Chapter 14 by Hellweg and coworkers discusses the considerable advantages of microemulsion-based decontamination systems with respect to practical boundary conditions and fundamental princi- ples of microemulsion formation. Additionally, the authors illustrate the further development to versatile, environmentally compatible and nonharmful systems containing nanoparticles and enzymes as active components. Several segments of the petroleum industry can be optimized with the use of microemulsions. Research has been carried out on potential microemulsified for- mulations for compression-ignition, cycle-diesel engines, which, in spite of bring- ing about a slight increase in consumption, produce less polluting emissions. In Chapter 15, Dantas Neto and coworkers summarize recent advances in microe- mulsions in this type of industry. Microemulsions can be considered as true nanoreactors, which can be used to synthesize nanomaterials. The main idea behind this technique is that by appropriate control of the synthesis parameters one can use these nanoreactors to produce tailor-made products down to a nanoscale level. Chapter 16 by Tojo and coworkers describes the use of Monte Carlo simulations to study the influence of the critical nucleus size and the chemical reaction rate on the formation of nanoparticles in micro- emulsions. Chapter 17 by Husein and Nassar focuses on exploring ways of maximizing the concentration of stable colloidal nanoparticles, nanoparticle uptake, in single (w/o) microemulsions. Chapter 18 by Ghosh describes the photophysical and interfacial electron transfer behavior of anatase TiO2 nanoparticles in microemulsions. Capillary electrophoresis is a powerful technique with relevant features of performance such as simplicity, versatility, very high resolution in short time 89595_C000.indd xxii 89595_C000.indd xxii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 27. Preface xxiii of analysis, and low cost of operation. The final chapter by Tripodi and Lucangioli describes the use of microemulsions in capillary electrophoresis as pseudostationary phases in the electrokinetic chromatography mode. This method has extensive applications in different fields of pharmaceutical analysis for the determination of drugs and their impurities in bulk material and pharmaceutical formulations for the dosage of drugs in biological fluids. In quintessence, this book represents the collective knowledge of young and renowned researchers and engineers in the field of microemulsions. This book covers recent advances in the characterization of the properties of microemulsions; it covers new types of materials used for the formulation and stabilization of microemulsions, and it also covers new applications. An important feature of this book is that the author of each chapter has been given the freedom to present, as he/she sees fit, the spectrum of the relevant science, from pure to applied, in his/her particular topic. Of course this approach inevitably leads to some overlap and repetition in different chapters, but that does not necessarily matter. Any author has his/her own views on, and approach to, a specific topic, molded by his/her own experience. I hope that this book will familiarize the reader with the scientific and engineering aspects of microemulsions, and provides experienced researchers, scientists, and engineers in academic and industry communities with the latest developments in this field. I would like to thank all those who contributed as chapter authors despite their busy schedules. In total, 52 individuals from 15 countries contributed to the work. All of them are recognized and respected experts in the areas they wrote about. None of them is associated with any errors or omissions that remain. I take full responsibility. Special thanks are due to the reviewers for their valuable comments as peer review is a requirement to preserve the highest standard of publication. My appreciation goes to Barbara Glunn of Taylor & Francis for her genuine interest in this project. Monzer Fanun Associate Professor Al-Quds University East Jerusalem, Palestine 89595_C000.indd xxiii 89595_C000.indd xxiii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 28. 89595_C000.indd xxiv 89595_C000.indd xxiv 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 29. xxv Editor Monzer Fanun is a professor in surface and colloid science, the head of the Colloids and Surfaces Research Laboratory, and a member of the Nanotechnology Research Group atAl-Quds University, East Jerusalem, Palestine. He has authored and coauthored more than 40 professional papers. He is a member of the European Colloid and Interface Society and a fellow of the Palestinian Academy for Science and Technology. In 2003, he received his PhD in applied chemistry from the Casali Institute of Applied Chemistry a part of the Institute of Chemistry at the Hebrew University of Jerusalem, Israel. 89595_C000.indd xxv 89595_C000.indd xxv 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 30. 89595_C000.indd xxvi 89595_C000.indd xxvi 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 31. xxvii Contributors Patricia A. Aikens BASF Corp. Stony Brook, New York Raid G. Alany School of Pharmacy University of Auckland Auckland, New Zealand Maria Carlenise Paiva de Alencar Moura Universidade Federal do Rio Grande do Norte Centro de Tecnologia Departamento de Engenharia Química UFRN—Federal University of Rio Grande do Norte Chemical Engineering Department Campus Universitário Natal, Brazil Levent Alparslan Department of Biopharmaceutics and Pharmacokinetics Faculty of Pharmacy University of Ege Izmir, Turkey and Center for Drug R&D and Pharmacokinetic Applications University of Ege Izmir, Turkey Hans-Juergen Altmann Armed Forces Scientific Institute for NBC Protection Munster, Germany Eduardo Lins de Barros Neto Universidade Federal do Rio Grande do Norte Centro de Tecnologia Departamento de Engenharia Química UFRN—Federal University of Rio Grande do Norte Chemical Engineering Department Campus Universitário Natal, Brazil F. Barroso Department of Physical Chemistry Faculty of Chemistry University of Vigo Vigo, Spain Pierre Bauduin Institut de Chimie Séparative de Marcoule Bagnols-sur-Cèze, France Marija Bešter-Rogač Faculty of Chemistry and Chemical Technology University of Ljubljana Ljubljana, Slovenia K. K. Bhasin Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India Liliana de Campo Department of Applied Mathematics The Australian National University Canberra, New South Wales, Australia 89595_C000.indd xxvii 89595_C000.indd xxvii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 32. xxviii Contributors Tereza Neuma de Castro Dantas Universidade Federal do Rio Grande do Norte Centro de Ciências Exatas e da Terra Departamento de Química UFRN—Federal University of Rio Grande do Norte Chemistry Department Campus Universitário Natal, Brazil A. Cid Department of Physical Chemistry Faculty of Sciences University of Vigo at Ourense Ourense, Spain Afonso Avelino Dantas Neto Universidade Federal do Rio Grande do Norte Centro de Tecnologia Departamento de Engenharia Química UFRN—Federal University of Rio Grande do Norte Chemical Engineering Department Campus Universitário Natal, Brazil M. de Dios Department of Physical Chemistry Faculty of Chemistry University of Vigo Vigo, Spain Gamal M. M. El Maghraby Department of Pharmaceutics King Saud University Riyadh, Saudi Arabia Monzer Fanun Faculty of Science and Technology Al-Quds University East Jerusalem, Palestine Stig E. Friberg Chemistry Department University of Virginia Charlottesville, Virginia L. García-Río Departamento de Química-Física Facultad de Química Universidad de Santiago de Compostela Santiago de Compostela, Spain Mirjana Gašperlin Faculty of Pharmacy University of Ljubljana Ljubljana, Slovenia Hirendra N. Ghosh Radiation and Photochemistry Division Bhabha Atomic Research Center Mumbai, Maharashtra, India Otto Glatter Institute of Chemistry University of Graz Graz, Austria D. Gómez-Díaz Departamento de Ingeniería Química Escuela Técnica Superior Universidad de Santiago de Compostela Santiago de Compostela, Spain Anja Graf School of Pharmacy University of Otago Dunedin, New Zealand 89595_C000.indd xxviii 89595_C000.indd xxviii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 33. Contributors xxix Alexandre Gurgel Universidade Federal de Viçosa Centro de Ciências Exatas e Tecnológicas Departamento de Química UFV—Federal University of Viçosa Chemistry Department Campus Universitário Viçosa, Brazil Thomas Hellweg Physical Chemistry I University of Bayreuth Bayreuth, Germany Maen M. Husein Department of Chemical and Petroleum Engineering University of Calgary Calgary, Alberta, Canada Andrej Jamnik Faculty of Chemistry and Chemical Technology University of Ljubljana Ljubljana, Slovenia Burçak Karaca Department of Medical Oncology School of Medicine University of Ege Izmir, Turkey Ercüment Karasulu Department of Biopharmaceutics and Pharmacokinetics Faculty of Pharmacy University of Ege Izmir, Turkey and Center for Drug R&D and Pharmacokinetic Applications University of Ege Izmir, Turkey H. Yesim Karasulu Department of Pharmaceutical Technology Faculty of Pharmacy University of Ege Izmir, Turkey Gurpreet Kaur Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India Khushwinder Kaur Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India F. N. Kolisis Institute of Biological Research and Biotechnology National Hellenic Research Foundation Athens, Greece and School of Chemical Engineering National Technical University of Athens Athens, Greece Karen Krauel-Goellner Institute of Food Nutrition and Human Health Wellington, New Zealand Werner Kunz Institute of Physical and Theoretical Chemistry University of Regensburg Regensburg, Germany 89595_C000.indd xxix 89595_C000.indd xxix 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 34. xxx Contributors Silvia Lucangioli Analytical Chemistry and Physicochemistry Department Faculty of Pharmacy and Biochemistry University of Buenos Aires Buenos Aires, Argentina S. K. Mehta Department of Chemistry and Center of Advanced Studies in Chemistry Panjab University Chandigarh, Panjab, India J. C. Mejuto Department of Physical Chemistry Faculty of Sciences University of Vigo at Ourense Ourense, Spain Satya Priya Moulik Center for Surface Science Department of Chemistry Jadavpur University Kolkata, West Bengal, India Nashaat N. Nassar Department of Chemical and Petroleum Engineering University of Calgary Calgary, Alberta, Canada V. Papadimitriou Institute of Biological Research and Biotechnology National Hellenic Research Foundation Athens, Greece Animesh Kumar Rakshit Department of Natural Sciences West Bengal University of Technology Kolkata, West Bengal, India André Richardt Armed Forces Scientific Institute for NBC Protection Munster, Germany Nilmoni Sarkar Department of Chemistry Indian Institute of Technology Kharagpur, West Bengal, India Debabrata Seth Department of Chemistry Indian Institute of Technology Kharagpur, West Bengal, India H. Stamatis Biological Applications and Technologies Department University of Ioannina Ioannina, Greece C. Tojo Department of Physical Chemistry Faculty of Chemistry University of Vigo Vigo, Spain Matija Tomšič Faculty of Chemistry and Chemical Technology University of Ljubljana Ljubljana, Slovenia Didier Touraud Institute of Physical and Theoretical Chemistry University of Regensburg Regensburg, Germany Valeria Tripodi Analytical Chemistry and Physicochemistry Department Faculty of Pharmacy and Biochemistry University of Buenos Aires Buenos Aires, Argentina 89595_C000.indd xxx 89595_C000.indd xxx 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 35. Contributors xxxi Stefan Wellert Physical Chemistry I University of Bayreuth Bayreuth, Germany A. Xenakis Institute of Biological Research and Biotechnology National Hellenic Research Foundation Athens, Greece Anan Yaghmur Institute of Biophysics and Nanosystems Research Austrian Academy of Sciences Graz, Austria 89595_C000.indd xxxi 89595_C000.indd xxxi 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 36. 89595_C000.indd xxxii 89595_C000.indd xxxii 11/10/2008 10:02:48 PM 11/10/2008 10:02:48 PM
  • 37. 1 1 A Phase Diagram Approach to Microemulsions Stig E. Friberg and Patricia A. Aikens CONTENTS 1.1 Introduction..................................................................................................1 1.2 Discussion ....................................................................................................2 1.2.1 Ordering–Disordering.......................................................................2 1.2.2 Temperature Dependence .................................................................7 1.2.3 Vapor Composition from Microemulsions ..................................... 10 1.3 Conclusion.................................................................................................. 14 Symbols and Terminologies................................................................................ 14 References........................................................................................................... 14 1.1 INTRODUCTION The phase diagram approach to microemulsions was introduced decades ago by Gillberg and collaborators [1]. At that time, it was not well received by the researchers in the area, because it emphasized that microemulsions are in fact micellar systems and the traditionally simplified thermodynamic treatment was very much in vogue at that time. Unfortunately, the ensuing arguments about the “true structure of microemulsions” shrouded the advantage of the approach, and it was only after the Israelachvili–Ninham analysis of the thermodynamics of such systems [2] that attention could be directed to the essential features of the phase diagram approach. A brief history of the development has been given by Lindman and Friberg [3]. In the following sections, the phase diagram approach will be applied to three attributes of microemulsions: (a) the importance of ordering versus disordering, (b) the temperature dependence of the behavior of microemulsions stabilized by polyethylene glycol adduct surfactants, and (c) the use of phase diagrams to obtain information on the composition of the vapor leaving microemulsion during its evaporation. 89595_C001.indd 1 89595_C001.indd 1 11/3/2008 10:21:55 AM 11/3/2008 10:21:55 AM
  • 38. 2 Microemulsions: Properties and Applications 1.2 DISCUSSION 1.2.1 ORDERING–DISORDERING The phase diagrams of microemulsions have traditionally been presented in two ways. The original one was built on the results of Ekwall on the association structures of amphiphilic systems [4] and was based on the associations in the water–surfactant combination. According to this approach, the development of the microemulsion structures was a result of the structural modifications brought about by the addition of less hydrophilic amphiphiles such as alcohols. The hydro- carbons in the microemulsions were considered solubilizates in this methodology and their effect on the structure was considered to be of secondary importance. The approach was very successful for W/O microemulsions, providing a simple tool for their formulation. A generic diagram is given in Figure 1.1. The essential feature of importance for the microemulsion is the fact that the inverse micellar solution and the aqueous solution of normal micelles are not in mutual equilibrium except for extremely low-surfactant concentrations. For higher-surfactant concentrations, the equilibrium is with the liquid crystalline phase. As a consequence, the transition from the normal micelles to inverse micelles (Figure 1.2) does not happen directly, but through a lamellar liquid crystal (Figure 1.3). W/O microemulsions stabilized by an ionic surfactant also employ a less hydrophilic amphiphile, which is known as the cosurfactant. The original cosur- factants were alcohols [5] and Gillberg realized early on [1] that W/O microemul- sions were obtained simply by adding a hydrocarbon to Ekwall’s inverse micellar solution (Figure 1.4). Addition of the hydrocarbon does not imply significant FIGURE 1.1 Partial generic phase diagram of a system water (W), surfactant (S), and medium chain length alcohol (A). (Adapted from Ekwall, P., in Advances in Liquid Crystals, Brown, G.H. (Ed.), Academic Press, New York, 1975, pp. 1–139. With permission.) A S W MS LLC IMS 89595_C001.indd 2 89595_C001.indd 2 11/3/2008 10:21:56 AM 11/3/2008 10:21:56 AM
  • 39. A Phase Diagram Approach to Microemulsions 3 FIGURE 1.2 In the aqueous solution micelle (left), the surfactant polar groups are orga- nized toward the surrounding water, while the hydrocarbon chains are inside the micelle. In an inverse micelle (right), the organization is opposite. Water Aqueous micelle Inverse micelle FIGURE 1.3 In a lamellar liquid crystal, water layers are separated by mirrored bilayers of surfactant. FIGURE 1.4 Addition of hydrocarbon to the inverse micellar solution (solid line) (Figure 1.1) gives a W/O microemulsion (hatched line). A/H S W 89595_C001.indd 3 89595_C001.indd 3 11/3/2008 10:21:56 AM 11/3/2008 10:21:56 AM
  • 40. 4 Microemulsions: Properties and Applications structure changes [6] and the W/O microemulsions were hence described as inverse micellar solutions. The approach was initially not received well by Schulman’s successors [7], and it is remarkable that Schulman’s initial publica- tion on the concept described these microemulsions as colloid solutions. The term “microemulsion” was coined much later [8]. The application of Ekwall’s presentation of phase diagrams offers several advantages. First, it provides an explanation of the fact that when the capacity to include water in a W/O microemulsion is exceeded, the phase appearing is not an aqueous liquid, but a lamellar liquid crystal. Secondly, it provides immediate clar- ification of the role of the cosurfactant. As demonstrated in Figure 1.5, the effec- tiveness of the cosurfactant depends decisively on its chain length. The difference in the sizes of the W/O microemulsion regions in Figure 1.5 demonstrates that decanol is far less useful as a cosurfactant than pentanol (if it is even useful at all). The explanation for this fact is not, as it may appear at a first glance, the difference in the stability of the inverse micelles; it rests with the fact that the shorter pentanol chain destabilizes the lamellar liquid crystal by disordering it, and so as a result, increasing the area for the inverse micellar solution. Following this approach, it would be logical to use butanol as a cosurfactant instead of pentanol, because its isotropic liquid region now expands continuously to the water corner (Figure 1.6). This large continuous isotropic liquid region at first appears highly appealing, but effective utilization of shorter chain length alcohols as cosurfactants is coun- tered by another factor. Butanol certainly destabilizes the lamellar liquid crystal efficiently (Figure 1.6), but when the hydrocarbon is added to form the micro- emulsion, the butanol is too water soluble and does not reach and reside at the oil/ water interface sufficiently. As a result, the system forms two separate phases: a traditional macroemulsion of oil and water. C5OH C10OH CnOH S W FIGURE 1.5 Comparison of the inverse micellar liquid areas for systems with pentanol and decanol. 89595_C001.indd 4 89595_C001.indd 4 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
  • 41. A Phase Diagram Approach to Microemulsions 5 The importance of the disordering action of the cosurfactant is confirmed by a later publication concerning O/W microemulsions [9]. In this case, the pentanol per se did not provide sufficient disordering effect as demonstrated by the features in Figure 1.7. The insufficient disordering is illustrated by the fact that the decane solubili- zation is limited and by the solubility gap along the sodium dodecyl sulfate (SDS)/W–C5OH axis. The latter is caused by the lamellar liquid crystal between the aqueous and pentanol solution (Figure 1.8). The addition of a hydrotrope, a more water soluble molecule with disordering action, supplemented the disordering and the liquid crystal range along the SDS/ W–C5OH axis disappeared (Figure 1.9) resulting in an excellent microemulsion area [10]. FIGURE 1.6 Isotropic liquid area for a system with butanol. C4OH Isotropic liquid S W FIGURE 1.7 Isotropic liquid in the partial phase diagram of decane, n-C10, pentanol, C5OH, and a solution of 15% SDS, in water. The liquid structure passes from an O/W microemulsion to a W/O one through a bicontinuous structure without a phase separation. SDS/W 15/85 I II O/W microemulsion W/O microemulsion Bicontinuous microemulsion N-C10 n-C5OH 89595_C001.indd 5 89595_C001.indd 5 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
  • 42. 6 Microemulsions: Properties and Applications The surfactant in this system is ionic, and hence salt has a similar action [11]. The ultimate extension of this action is amply exemplified in the early publications from the field of microemulsion-assisted petroleum recovery [12]. The phase diagram approach to microemulsions following Ekwall [1] is characterized by a section through the three-dimensional diagram according to Figure 1.10a. Alternative publications with different sectioning (Figure 1.10b) have also gained popularity [13]. Both these presentations are useful; the second one suffers from the disadvantage of not catching the strong variation in the areas with the surfactant/cosurfactant ratio as accentuated by Figure 1.8. FIGURE 1.8 Part of the phase diagram water (W), SDS, and pentanol (C5OH). The areas named microemulsions in this Figure 1.4 were called micellar solutions in Ekwall’s terminology (From Ekwall, P., in Advances in Liquid Crystals, Brown, G.H. (Ed.), Academic Press, New York, 1975, pp. 1–139. With permission.) SDS W Bicontinuous microemulsion Lamellar liquid crystal C5OH W/O microemulsion O/W microemulsion FIGURE 1.9 Microemulsion region in the system water/SDS/sodium xylene sulfonate, W/SDS/SXS, pentanol, C5OH, and decane, C10. C5OH C10 W/SDS/SXS 80.9/14.3/4.8 89595_C001.indd 6 89595_C001.indd 6 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
  • 43. A Phase Diagram Approach to Microemulsions 7 1.2.2 TEMPERATURE DEPENDENCE It is seen above that the areas for microemulsions stabilized by ionic surfactants are decisively dependent on the structure of the cosurfactant to cause the necessary disorder in the system. Microemulsions stabilized by polyethylene glycol adduct nonionic surfactants, on the other hand, are characterized by the fact that cosurfactant is not used. Instead, the areas of stability now rely on temperature (Figure 1.11) although the relation with the liquid crystal structure is still the essential element [14]. The main theme of this dependence is illustrated in Figures 1.12 and 1.13 [15], which show the generic phase diagram for an alkyl ether surfactant with an aliphatic hydrocarbon of a moderate length (approx. 12 carbons), and a short polyethylene glycol chain (approx. 4 ethylene glycol units). First, the diagram is characterized by a complete disparity of the solubility of the surfactant in water FIGURE 1.10 Two main representations of the microemulsion pseudophase diagram. The left depiction (a) is the Ekwall–Gillberg approach, which treats the hydrocarbon/ cosurfactant liquid as one component, while the right model (b) combines the surfactant and cosurfactant into one component. W S Co-S H W S Co-S H (b) (a) FIGURE 1.11 Phase equilibria for the system water (W), a polyethylene glycolalkyl ether (S) and an aliphatic hydrocarbon (H). Low-temperature behavior is depicted in the upper left-hand diagram, high-temperature features are depicted in the lower right-hand diagram. W S H S W H 89595_C001.indd 7 89595_C001.indd 7 11/3/2008 10:21:57 AM 11/3/2008 10:21:57 AM
  • 44. 8 Microemulsions: Properties and Applications and hydrocarbon with a moderate rise in temperature to 75°C.At lower temperature, the solubility is restricted to water transitioning into an aqueous micellar solution, followed by a lamellar liquid crystal, and finally an inverse micellar liquid with increasing surfactant content. At a higher temperature, the surfactant solubility is restricted entirely to the hydrocarbon, without any ordered structures. W H.S. H FIGURE 1.12 Features of the system at the HLB temperature. FIGURE 1.13 Three next stages after the state at left in Figure 1.11. S H W O/W microemulsion W Internal two-phase W 89595_C001.indd 8 89595_C001.indd 8 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
  • 45. A Phase Diagram Approach to Microemulsions 9 In between these two temperature extremes, lies the hydrophilic–lipophilic balance (HLB) temperature. Here, the aqueous micellar phase has disappeared or is severely restricted (Figure 1.13), and a liquid phase is found instead at a water/ hydrocarbon ratio of approximately 1 with a moderate concentration of surfactant. This liquid is a microemulsion with a bicontinuous structure. From a phase diagram point of view, the development of the features from those in the left-hand diagram of Figure 1.11 to those in Figure 1.12 is of interest because of the rather intricate details of the equilibria. Figure 1.13 presents the initial changes in greater detail. The first development from the stage in the left-hand diagram of Figure 1.11 is that the hydrocarbon and the surfactant become mutually soluble and the lamellar liquid crystal is extended toward high-hydrocarbon content. These two areas remain approximately constant during the next stages. In the first of these (top right, Figure 1.13), the micellar region along the water–surfactant axis is limited with respect to the maximum water content. To reach the water corner, a certain ratio of hydrocarbon to surfactant is required. The effect is that a two-phase region is formed extending from the water corner. In the next step (bottom right, Figure 1.13), the two regions are separated and the one with highest hydrocarbon to surfactant ratio forms an O/W microemulsion, which has the specific property of being infinitely dilutable with water. It should be noted that the O/W microemulsions stabilized by an ionic surfactant such as the one in Figure 1.9 cannot be diluted with water. Any such attempt leads to phase separation and a macroemulsion is formed. The fact that there is no equilibrium between the O/W microemulsion and the remnant of the micellar solution is a remarkable feature in the diagram. Instead, the two liquids are in equilibrium with pure water and with the lamellar liquid crystal. Further progression toward the HLB temperature pattern in Figure 1.12 is depicted in Figure 1.14. Now the O/W microemulsion area is separated from the water corner, but forms an unconnected phase in the water-rich part of the system. The ensuing phase equilibria are depicted in the enlarged partial diagram on the left-hand side of Figure 1.14. The complexity of the equilibrium conditions are well illustrated by the FIGURE 1.14 Subsequent step to the configurations in Figure 1.13. S H W W 89595_C001.indd 9 89595_C001.indd 9 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
  • 46. 10 Microemulsions: Properties and Applications number of phases (MS, aqueous micellar solution; O/W, O/W microemulsion; LC, liquid crystal; O, hydrocarbon–surfactant liquid) found when adding hydrocarbon to an aqueous solution of surfactant less concentrated than the separated phase. The sequence isW + MS →W + MS + LC →W + LC →W + LC + O/W →W + O/W → W + O/W + O→W + O; an extraordinary number of seven combinations for surfactant concentration less than that of any association structure phase. The bicontinuous microemulsion region coalesces with the hydrocarbon–sur- factant liquid area forming a W/O microemulsion region reaching toward greater fractions of water. The surfactant–hydrocarbon ratio for maximum water solubiliza- tion depends on the maximum solubilization of the hydrocarbon into the lamellar liquid crystal. With increasing temperature, the solubilization capacity is reduced and the maximum for water solubilization into the W/O microemulsion is shifted to greater surfactant–hydrocarbon ratios. The final result of this trend is the system of the hydrocarbon–surfactant liquid in equilibrium with water, in accordance with the right part of Figure 1.11 (Figure 1.15E). 1.2.3 VAPOR COMPOSITION FROM MICROEMULSIONS The recently introduced algebraic method to extract information from phase diagrams [16] has been used to quantify evaporation from microemulsions [17]. The approach as such does not provide additional information to the experimentally determined phase diagram, but introduces a system to illustrate the influence of the relative humidity (RH) on the direction and volume of the evaporation and its path that is not immediately available otherwise. H W H.S. A H W H.S. C H W H.S. B H W H.S. E H W H.S. D FIGURE 1.15 Subsequent patterns between that of the HLB temperature in Figure 1.12 and the final appearance to the right in Figure 1.11. (Reproduced from Shinoda, K. and Friberg, S.E., Emulsions and Solubilization, John Wiley & Sons, New York, 1986.) 89595_C001.indd 10 89595_C001.indd 10 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
  • 47. A Phase Diagram Approach to Microemulsions 11 The method basically relates the changes in the composition of the liquid phase to the composition of the released vapor. This information would not be of practical value if the evaporation took place in completely dry air or in vacuum; the difference in the composition of the liquid phase obviously equals the compo- sition of the escaping vapor. In addition, if the evaporation were to take place under conditions close to equilibrium, the partial vapor pressures could be utilized to calculate the activities of the components in the liquid phase. Evaporation how- ever, usually takes place into an atmosphere at a certain level of humidity and this fact affects the interaction between the liquid phase and its vapor. This influence is conveniently attended to using the algebraic approach as illustrated by an example [17]. The system to be discussed consists of water (W), cosurfactant (C), and surfactant (S) and is depicted in Figure 1.16. The composition of the discharged vapor from a weight fraction composition (W1, C1, and S1) is obtained from the tangent to the experimentally determined evaporation path, which is given a function C(S). 1 1 1 1 ) ( d /d C C S S C S = + − (1.1) Setting S = 0, the composition of the released vapor (WV, CV, and 0) is found. V 1 1 1 1 ( / ) d d C C S C S = − (1.2) and V 1 1 1 1 1 d / ) d ( W C S C S = − + (1.3) FIGURE 1.16 System used is the W/O microemulsion base in which the cosurfactant is a volatile compound, illustrating the behavior of a hydrocarbon. W S C Evaporation composition (W1, C1, and S1) Composition of released vapor (Wv, 0, and Sv) 89595_C001.indd 11 89595_C001.indd 11 11/3/2008 10:21:58 AM 11/3/2008 10:21:58 AM
  • 48. 12 Microemulsions: Properties and Applications This is the composition of the released vapor as obtained from experimental results. These are inherently affected by the RH of the receiving atmosphere with the actual vapor phase including the indigenous atmospheric water (WRH). V 1 1 1 1 RH 1 d / ( ) d W C S C S W = − + + (1.4) Realizing that the contribution from the released microemulsion vapor to the total water in the vapor is proportional to its evaporation rate and assuming ideal behavior of the vapor, the weight fractions of the two volatile compounds in the released vapor become V C C C C W W W { ( /[ Em) 0.01RH 0)} ( ] C P M P M M P P = + − μ (1.5) and V W W W C C W W W { ( ( { ( Em) 0)}/[ Em) 0.01RH 0)}] ( W M P P P M M P P = μ − + μ − (1.6) where P is pressure M is molecular weight As a contrast the equilibrium values for the receiving atmosphere are V C C C C W W /[ ( Em)] C P M P M M P = + μ (1.7) and V W W C C W W Em)/[ ( Em)] ( W M P P M M P = μ + μ (1.8) The combination of these equations with the phase diagram offers a great deal of insight on the evaporation [17]. A few salient points will be examined here. At first, the indigenous vapor pressure of water in the atmosphere will exceed the vapor pressure from the microemulsion for sufficiently high values of RH. As a consequence, the evaporation of water is now reversed; it is absorbed into the microemulsion liquid, as quantified by the general relation of the ratio between evaporating water and volatile organic compound, a first-order linear equation: C C W W W C C { ( ( [ Em) 0.01RH 0)}]/ R P M M P P P M = + μ − (1.9) The reversal of direction happens when the RH exceeds the value obtained in Equation 1.10: W W RH Em)/0.01 0 ( ) ( P P > μ (1.10) Figure 1.17 illustrates this change of course in the phase diagram (negative CV). 89595_C001.indd 12 89595_C001.indd 12 11/3/2008 10:22:00 AM 11/3/2008 10:22:00 AM
  • 49. A Phase Diagram Approach to Microemulsions 13 With realistic values of the vapor pressures and the molecular weights, such as PC = 0.1 PW(0) = 20 PW(μEm) = 10 MW = 18 MC = 150 Equation 1.9 becomes / 12 0.24RH W C = − (1.11) and the reversal takes place at an RH of 50%. The RH has another critical point representing the value at which the reduction in the entire weight of the microe- mulsion with evaporation is reversed. The general expression is W W C C W W RH ( Em) /0.01 ( ) 0) ( M P P M M P = μ + (1.12) This represents a slightly greater value than that for the reversal of the water transport direction in Equation 1.10 with a numerical value of the RH in this case W S C (W1, C1, S1) Composition of released vapor (Wv, 0, and Sv) x Water evaporating Water absorbed No water evaporation FIGURE 1.17 Phase diagram illustrating the change in water evaporation direction (arrows from the evaporation composition, W1, C1, and S1) with increased RH. The evapo- ration changes from water evaporating to water being absorbed into the liquid from the atmosphere. 89595_C001.indd 13 89595_C001.indd 13 11/3/2008 10:22:03 AM 11/3/2008 10:22:03 AM
  • 50. 14 Microemulsions: Properties and Applications being 54%. In what may be considered an oddity, this value is also the one at which the value of CV for the released vapor instantaneously goes from positive to negative infinity with increased RH. 1.3 CONCLUSION Phase diagrams are shown to provide valuable information on the role that struc- ture of the surfactant, cosurfactant, and oil plays in determining the properties of the system at any composition. In addition, it is demonstrated that degree of order/ disorder of lamellar liquid crystalline phases within a system stabilized by an ionic surfactant is determined by the chemical structure of the components and this in turn influences the magnitude, the nature (O/W, bicontinuous, W/O), and the location of the microemulsion phases. When nonionic polyethylene glycol ether surfactants are used instead of ionic ones, the temperature of the system is the crucial determining factor, with optimum properties at the HLB temperature of the surfactant. Finally, it is shown that straightforward algebraic analysis can be used to determine the composition of the evaporating gas from the microemulsion under practical use conditions by taking into account the atmospheric RH, utilizing the vapor pressures of the volatile components, molecular weights, and extrapolations from the phase diagram. SYMBOLS AND TERMINOLOGIES W water O oil C cosurfactant H hydrocarbon A alcohol μEm microemulsion RH relative humidity P pressure M molecular weight REFERENCES 1. Gillberg, G., Lehtinen, H., and Friberg, S. E. (1970). IR and NMR investigation of the conditions determining the stability of microemulsions, J. Colloid Interf. Sci. 33, 40–49. 2. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W. (1976). Thermodynamics of amphiphilic association structures, J. Chem. Soc. Faraday Trans. II 72, 1525–1533. 3. Lindman, B. and Friberg, S. E. (1999). Microemulsions—a historical overview, in Mittal, K., Kumar, P. (Eds), Handbook of Microemulsions Science and Technology, Marcel Dekker, New York, pp. 1–12. 4. Ekwall, P. (1975). Composition, properties and structures of liquid crystalline phases in systems of amphiphilic compounds, in Brown, G. H. (Ed), Advances in Liquid Crystals, Academic Press, New York, pp. 1–139. 89595_C001.indd 14 89595_C001.indd 14 11/3/2008 10:22:04 AM 11/3/2008 10:22:04 AM
  • 51. A Phase Diagram Approach to Microemulsions 15 5. Hoar, T. P. and Schulman, J. H. (1943). Transparent water-in-oil dispersions: The oleopathic hydro-micelle, Nature 152, 102–103. 6. Sjoeblom, E. and Friberg, S. (1978). Light-scattering and electron microscopy deter- minations of association structures in W/O microemulsions, J. Colloid Interf. Sci. 67, 16–30. 7. Prince, L. M. (1969). A theory of aqueous emulsions II: Mechanism of film curva- ture at the oil/water interface, J. Colloid Interf. Sci. 29, 216–221. 8. Schulman, J. H., Stoeckenius, W., and Prince, L. M. (1959). Mechanism of forma- tion and structure of micro-emulsions by electron microscopy, J. Phys. Chem. 63, 1677–1680. 9. Friberg, S. E., Brancewicz, C., and Morrison, D. (1994). O/W microemulsions and hydrotropes: The coupling action of a hydrotrope, Langmuir 10, 2945–2949. 10. Friberg, S. E. (1997). Hydrotropes, Curr. Opin. Surf. Colloid Sci. 2, 490–494. 11. Friberg, S. E. and Buraczewska, I. (1978). Microemulsions in the water–potassium oleate–benzene system, Prog. Colloid Polym. Sci. 63, 1–9. 12. Bourrel, M. and Schechter, R. S. (1988). Microemulsions and Related Systems, Marcel Dekker, New York. 13. Bauduin, P., Touraud, D., and Kunz, W. (2005). Design of low-toxic and tempera- ture-sensitive anionic microemulsions using short propyleneglycol alkyl ethers as cosurfactants, Langmuir 21, 8138–8145. 14. Stubenrauch, C., Frank, C., Strey, R., Burgemeister, D., and Schmidt, C. (2002). Lyotropic mesophases next to next to highly efficient microemulsions: A 2H NMR study, Langmuir 18, 5027–5039. 15. Shinoda, K. and Friberg, S. E. (1986). Emulsions and Solubilization, John Wiley & Sons, New York. 16. Friberg, S. E. (2006). Weight fractions in three-phase emulsions with an Lα phase, Colloids Surf. A. 282/283, 369–376. 17. Friberg, S. E. and Aikens, P. A. submitted to J. Disp. Sci. Tech. 89595_C001.indd 15 89595_C001.indd 15 11/3/2008 10:22:04 AM 11/3/2008 10:22:04 AM
  • 52. 89595_C001.indd 16 89595_C001.indd 16 11/3/2008 10:22:04 AM 11/3/2008 10:22:04 AM
  • 53. 17 2 Physicochemistry of W/O Microemulsions: Formation, Stability, and Droplet Clustering Animesh Kumar Rakshit and Satya Priya Moulik CONTENTS 2.1 Introduction................................................................................................ 17 2.2 Basics of Formation ................................................................................... 21 2.3 Stability of Microemulsion ........................................................................ 22 2.3.1 Method ......................................................................................... 22 2.3.2 Droplet Dimensions ..................................................................... 32 2.3.3 Energetics of Droplet Clustering .................................................34 2.3.4 Dynamics of Dispersed Droplets................................................. 36 2.3.5 Conductance of Microemulsions ................................................. 36 2.3.6 Useful Percolation Equations....................................................... 39 2.3.7 Conductance Percolation and EMT ............................................. 41 2.3.8 Viscosity Percolation ................................................................... 43 2.3.9 Antipercolation and Double Percolation......................................44 2.3.10 Activation Energy for Percolation................................................ 45 2.3.11 Additive Effects on Percolation....................................................46 Acknowledgments............................................................................................... 49 Symbols and Terminologies................................................................................ 49 References........................................................................................................... 51 2.1 INTRODUCTION Microemulsions are microheterogeneous, thermodynamically stable mixtures of oil and water. Here, the term “oil” means any water insoluble organic liquid. Macroscopically, they are homogeneous systems. Such oil/water disperse sys- tems were known for a long time as there were some commercial floor cleaning products available in the American market at the turn of the twentieth century. 89595_C002.indd 17 89595_C002.indd 17 11/3/2008 10:32:20 AM 11/3/2008 10:32:20 AM
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  • 55. some of the big ones.” “I think I can get another like yours right here at shore,” said Tom, and he threw in. Shortly he had a bite, and almost duplicated Jack’s catch. Meanwhile Jack was cleaning and scaling his prize, and drying himself out. The other boys had fair luck with rod and line, and then it was up to Dick to cook the fish, which he did, frying them in bacon and corn meal. “Oh, say, maybe they don’t smell good!” cried Tom, as the savory odor was spread about the camp. “They’ll taste better,” was Jack’s comment. The evening meal was a great success and they all voted that Dick was a much better cook than he had given himself credit for. “How are you on pies?” asked Tom, as they sat around the campfire that evening, after everything was ready for bed. “Think you can tackle them, Dick? We’ve got prepared flour, and you can use some jam, or canned apples, for filling.” “I’ll try it,” agreed the amateur cook. “We’ll have pie to-morrow.” They did not sleep very well that night, as the beds were rather hard, not having been properly made, and they were all rather excited over the events of the day. Breakfast, however, with coffee, and bacon and eggs which they had brought from Wilden, put them all in good humor, and they made a merry meal. “Now for some fishing!” exclaimed Tom, as he went down to look about his motorboat. “And I’m going to take a gun and see if I can get anything in the line of game,” put in Jack. “It’s out of season for most things, but I may get something in the bird line.”
  • 56. “And Dick is going to make pie,” said Bert. “Make four, old man, so there’ll be one apiece.” “All right,” agreed the young cook good naturedly. “I won’t guarantee results, but I’ll do my best.” Tom started out in the boat with Bert to do some fishing, while Jack wandered off in the woods with his shotgun. Dick did up the dishes and then began rummaging around in the supplies. Soon he was whistling away and, as Tom and Bert could see, from where they were in the boat, he was kept quite busy over something. “Well, did you get ’em made?” asked Tom, when they had all assembled for dinner. “How about the pies, Dick?” “There they are,” was the retort, and Dick pointed to the pastry. “Hum! They smell good!” exclaimed Jack, as he whiffed an odor from the pies. “They look good,” commented Tom. “Let’s see if they taste good,” suggested Bert. The pies were served as dessert, and at the first mouthful Tom let out a howl. “For the love of tripe!” he cried. “What did you put in these pies, Dick?” “Apples, of course,” replied the injured cook. “What did you suppose it was?” “Well, if those are apples then they’re flavored with something funny,” declared Tom. “Where’s the can you used?” Dick brought two empty tin cans up to the table, which was made from packing boxes. “The paper labels soaked off,” he explained, “but there were cans of apples on top and below these so I thought it was all right. Isn’t it?” Tom took a smell, and cried out:
  • 57. “Say, fellows, he dumped a can of quinces in the apple pie stuff and baked that all together and then used baking soda for powdered sugar! Oh wow! What a taste!” There was a general laugh, and Dick replied with: “Well, if you fellows think you can do any better you can have my job. I’m sick of being cook.” “Tut, tut! It’s all right,” said Tom hastily. “We were only fooling. You’re doing fine, Dick, only, after this, smell of a can if it hasn’t got a label pasted on it, and taste the powdered sugar.” But if the pies were a failure, the rest of the dinner was good, and later on Dick proved that he could make good pastry when he used the right ingredients. They had more fish that day, as luck was good, but the game was scarce, as might have been expected at that season of the year. After dinner, the rest of the day was spent in getting the camp into better shape, and making the beds more comfortable. The boys were in the habit of making up a camp fire early in the evening, and sitting in the glow of it to talk. They did this on their second night, and when it had about died down Tom tossed on some heavy sticks of wood and remarked: “Well, I’m going to turn in. I’m tired and I want some sleep. To- morrow we’ll take a long boat ride.” “When are we going to the old mill?” asked Jack. “Oh, maybe we can try that soon if we like,” said Tom. It was nearly midnight, as Tom ascertained by looking at his watch, when he was suddenly awakened by hearing something moving about near the sleeping tent. At first he thought it was one of his chums, and he called out: “Who’s that? You, Jack?”
  • 58. There was no answer, and, looking across to the other cots, our hero saw the forms of his companions under the covers. They were all quiet. “There’s some one out there,” he murmured. Rising cautiously he stepped to the flap of the canvas shelter and peered out. In the dying glow of the camp fire he saw an old man silently walking toward the tents. “For gracious sake!” breathed Tom to himself. “If that isn’t the old hermit of the mill I’m a lobster! I wonder what he’s doing here?” With anxious eyes he watched, and as the moon came out from behind a cloud, to add to the glow of the campfire, Tom saw the light glint on a gun. “He’s looking for us!” whispered Tom. “I wonder what I’d better do?”
  • 59. CHAPTER VIII OLD ACQUAINTANCES For a moment the lad stood there at the flap of the tent, pondering over the situation. He realized that he might have a desperate character to deal with—a man who would not listen to reason, and who was impulsive, as evidenced by his leap into the water after the motorboat. “But I’ve got to do something,” thought Tom. “If I don’t he may take a shot at us, not meaning to do any harm, but just because he’s erratic. And that sort of a bullet does just as much harm as any other. If he should fire into the tent——” Tom did not finish out his thought, for at that moment there was a movement on the part of the old man. He had been standing still, silently regarding the camp, and now he again advanced. “He’s going to see what sort of a place we have here,” mused Tom. “I wonder if I’d better awaken the boys?” He thought it over for a moment and then decided that perhaps he could best deal with the old man alone. “But how?” he asked himself. Tom watched the hermit. He came on with a tread like that of a cat—silently—stealthily—peering from side to side. At times he muttered to himself. “I’ll see if I can take him by surprise,” decided Tom. Stepping back, where he could not be seen, inside the tent, our hero suddenly yelled:
  • 60. “Get out of here! What are you doing in our camp? Be off before I set the dogs on you!” The old man was evidently startled. He stiffened as he stood, but Tom was glad to see that he did not bring the gun to bear. From under the shaggy eyebrows the hermit gazed about him as if to determine whence came the voice. But if Tom had any idea that he could frighten the man into going away he was mistaken. For the hermit of the mill came forward until he stood directly in front of the big tent, and then, straightening up, he fairly shouted: “Ha! I have found you; have I? Those who brought their infernal puffing engine on my lake. Now you are in my woods. I have been looking for you. I warn you away! You must leave at once! I will not be cheated out of my fortune this way. Leave my woods or it will be the worse for you!” and he shook his fist at Tom, who had now stepped into view at the flap of the tent. “Hello! What’s the row?” called Jack, suddenly awakening. “Is the camp on fire?” asked Dick. “What’s wrong, Tom?” cried Bert, and all three of our hero’s chums sprang from their cots and crowded around him. “It’s our old friend the hermit of the mill,” explained Tom in a low voice. “He’s come to drive us out of the woods.” “What are you going to do?” asked Jack. “I don’t know. Let’s see what he does.” “He may be dangerous,” commented Dick. “And these may be his woods,” added Bert. “Nonsense,” declared Tom. “I asked dad about it before I came up, and he said this part of the forest belonged to a big lumber company that was holding it for the trees to get bigger before cutting. This old man doesn’t own it any more than we do.”
  • 61. “Then you’re going to stick?” “I sure am!” During this talk the old hermit remained motionless, regarding the boys with angry eyes. Then he spoke again. “Well, are you going to take yourselves out of my woods? Are you going to leave at once? I demand that you go!” “No, sir, we are not going,” declared Tom, firmly but respectfully, for after all, he thought the age of the man was entitled to some deference. “You must leave my woods!” the hermit insisted. “I have been bothered enough in the search for the fortune hidden from me. I want to be alone in my woods. Go!” and he pointed his finger toward Wilden. “I do not think you have the right to make us go,” said Tom. “I understand these are not your woods, and we have as good a right to camp here as you have to wander about. We are not going!” For a few seconds the old man seemed dazed at the bold answer. Probably he had expected a meek compliance, but, as it developed, Tom’s answer was the best that could have been given. Pausing a moment the hermit gazed almost reproachfully at the lads and then, with another shake of his fist at them, he called: “Well, you have been warned, and now you must take the consequences. The price of your folly is on your own heads!” He turned and vanished into the shadows of the woods. “Whew! Quite dramatic!” exclaimed Tom, as he turned to his chums. “I should say so,” agreed Jack. “Nice thing to be awakened from pleasant dreams and told to move on in a trackless forest at midnight. He’s as bad as Professor Skeel used to be.”
  • 62. “Speaking of Skeel reminds me,” observed Tom. “Do you think he has come up here to camp?” “Hard to say,” murmured Bert. “But I know one thing, as long as I’m awake I’m going to have something to eat. Are there any of those chicken sandwiches left, Dick?” “I guess so. And there’s some cold tea.” “Warm it up then, and we’ll have a lunch.” “Say, what do you think this is; a quick-eat restaurant?” asked the amateur cook. “Oh, go ahead,” suggested Tom. “We’ll all help, and maybe we’ll get to sleep again, after this interruption, if we eat.” The oil stove was lighted, and the tea put on to warm, while Dick set out a plate of sandwiches he had made from canned chicken. Then the boys ate and talked. “That old hermit is sure on our trail,” declared Tom. “But he doesn’t seem to be as dangerous as the folks made out,” commented Jack. “I guess he’s just simple-minded, thinking of the treasure in the old mill,” added Bert. “By the way, Tom, when are we going to visit the ruins, and have a try for the buried gold?” and he laughed. “Oh, we’ll go over there some time,” agreed Tom. “I’d like to pick a day, though, when old Wallace wouldn’t be on hand. I’m not exactly afraid of him, but, from what I can understand, he does own the mill, though not these woods, and if he ordered us off that property we’d have to go.” “But we can take a chance,” suggested Dick. “Oh, sure,” came from Tom. “Say, but that old chap must spend all his time wandering about the woods. I wonder where he sleeps when he’s away from the mill?”
  • 63. “Oh, he probably has plenty of bunks and caves that we never would dream of,” said Jack. “Well, I’m going to turn in,” he added, with a yawn. “If he comes back again kindly tell him, Tom, to wait until morning before doing any more ordering-off.” Once more the lads sought their cots, to sleep undisturbed until morning. The day was spent in getting their camp more in ship- shape, and getting in a supply of wood for camp fires, and for cooking in case their oil gave out, or the portable stove failed. In the afternoon they went fishing, and had good luck. Though they kept watch for the hermit, they did not see him. The woods and lake were as deserted as though they were in some country as yet unvisited by man, and there were no evidences that any camping parties had ever visited the region where the boys were. “It sure is wild,” said Jack, as he gazed about. “It’s just the cheese though,” declared Tom. “We couldn’t have picked out a better place.” “And as soon as we get busy on the secret of the old mill there may be lots of happenings,” added Bert. A week passed, during which our friends enjoyed life to the utmost. They fished, and as the lake had seldom been visited by devotees of the rod and line it proved a garden spot for such sport. One had but to throw in a line to have a bite. They hunted, too, but as the season was not open they managed to kill only a few foxes and skunks, and, as their fur was not of much value in the summer, even this they gave up as rather unprofitable work. “It’s the mill we want to head for,” insisted Jack. “Come on, Tom, let’s get up an expedition and go there. We can go in the boat, for, as you say, the mill is on the river that runs into the lake. Come on.” “All right, we’ll go to-morrow,” agreed Tom. Accordingly, having set their camp to rights, and having put up a lunch, for they would not be back to dinner, they set off in the Tag, heading up the lake to where the river entered it.
  • 64. “She’s running better than she did at home,” remarked Dick to Tom, as he looked at the puffing motor. “Yes, but don’t say anything,” cautioned our hero. “She may be holding back for a kick-up. I never praise this motor, for I actually believe it knows what you say. Let well enough alone,” and the others laughed at his quaint conceit. It was a beautiful day, and the trip along the lake was much enjoyed. It was rather lonesome, but the boys did not mind that. As they moved along the shore of a little cove Jack suddenly called: “Hold on! I think I heard something moving near the bank there,” and he pointed just ahead. “Slow down the engine,” called Tom to Dick, and the latter throttled down, making the machinery almost noiseless. Then they all heard a crashing in the underbrush. “Maybe it’s the hermit,” suggested Bert. “Very likely,” agreed Jack. “I hope he doesn’t begin on one of his tantrums again.” The sounds in the bushes grew, and a moment later three figures suddenly stepped into view on the sandy beach of the lake. “Look!” exclaimed Tom in a low voice. “If this isn’t the limit!” All four boys gazed toward the figures, to behold their old acquaintances, Professor Skeel, Sam Heller and Nick Johnson!
  • 65. CHAPTER IX AT THE OLD MILL Difficult it would be to say which party was the more surprised. Certain it was, though Tom and his chums knew that their former teacher intended coming to the vicinity, and though they realized that Sam and Nick had gotten off the train with camping stuff near Wilden, they never expected to meet the three in this spot. And, for that matter, neither did Mr. Skeel and the two lads, with whom he seemed to be on friendly terms, think to behold Tom, for the former plainly showed the surprise he felt. “Well what do you know about this?” asked Jack, in a low voice. “It’s the limit,” agreed Tom. “Mind your wheel or you’ll have us on shore,” said Bert. “There’s a big rock just ahead of you.” Tom shifted the wheel with a rapid turn. He had been so interested in looking at the trio on shore that he had not noticed where he was steering. “Shall we speak to ’em?” asked Jack. “No, don’t,” advised Bert. “There’s no use getting into an argument.” “And yet we might find out something about them, and what they are doing up here,” insisted Jack, who generally liked to take the initiative. “I guess we’d better not,” spoke Tom. “Anyhow, they wouldn’t give us any satisfaction. If they hail us we’ll answer, and that’s all.”
  • 66. But the three on shore evidently had no intention of speaking. After his first stare of surprise Mr. Skeel was seen to speak to Sam and Nick, and then, with a final glance at our friends, the trio turned and plunged back into the woods. “Well, that’s over—for the time being,” remarked Dick. “Yes,” assented Bert. “Can you see which way they’re going, Tom?” “Why should we want to?” “Because they may be going to the same place we are.” “What, to the old mill?” “Sure.” “They don’t know anything about it,” declared Tom. “How do you know? That story of buried treasure is more or less known all over this section, and the hunt old Wallace is making for it, too. So why shouldn’t Mr. Skeel, and Sam or Nick know of it?” “Well, maybe you’re right,” agreed Tom. “But we can’t see which way they’re headed. The brush is too thick.” “We’re not far from the mill, if I’m any judge,” said Jack. “Why?” Tom wanted to know. “How can you tell? You’ve never been there.” “No, but there’s a current setting into the lake now, and that means the river isn’t far off. The mill is on the river, so, naturally we’re near the mill. Q. E. D., as we used to say after we’d floundered through a geometry proposition.” “Well, maybe you’re right,” admitted our hero. “Another thing,” went on Jack. “If we’re near the mill, so are those fellows. So you see——” “By Jove!” cried Tom. “I shouldn’t be surprised but what you were right, Jack. This man Skeel would be up to any proposition to make
  • 67. money, and he may, as you say, have heard the rumor of treasure in the old mill.” “How do you account for him meeting Sam and Nick?” asked Bert. “Oh, it probably just happened,” suggested Tom. “If they are camping near here, and Skeel is doing the same thing, it’s not out of reason that they should meet. Well, if they’re after the treasure in the old mill I don’t see what’s to prevent us having a go for the same thing.” “If Old Wallace will let us,” put in Bert. “Oh, well, we’ll have to take a chance with him,” said Tom. “We’ll have to wait until he’s away from home, which he seems to be most of the time.” “And if we get the treasure, what will we do with it?” inquired Dick. “Wait until we do,” laughed Tom. “I don’t believe there is one chance in a thousand of there being any treasure there, and if there is, it’s a hundred to one shot that we can’t find it, nor can anyone else. But it will be fun to have a go for it.” “And if we do find it,” put in Jack, “we’ll all take a trip to Europe.” “No,” spoke Tom, quietly, “if we do find any treasure, it will have to go to the one who owns it—the old hermit, very likely.” “Oh pshaw!” cried Jack. “After the mean way he treated us, Tom?” “Sure. Right is right. But say, don’t let’s get into an argument over such a remote possibility. Wait until we get to the mill, and have a look around. I’m an expert on buried treasure, and I can tell, as soon as I see a place, what the prospects are,” and Tom’s chums joined in his hearty laugh. “Well, speed up,” suggested Jack, “and we’ll see what sort of an Eldorado lies before us. Westward ho!” and he struck a dramatic attitude.
  • 68. Tom turned on more gasolene and advanced the spark, so that the Tag shot ahead. There was no further sign of Professor Skeel and the two boys. “There’s the river!” exclaimed Bert, about a quarter of an hour later, as the boat went around a bend, and they came into view of a stream flowing into the lake. It was as wild and picturesque as the lake itself, with big trees on either bank, overhanging the water in places. “Say, that’s great!” cried Tom. “I’m going to get some pictures of that. Take the wheel, Jack, while I get out my camera.” Tom was soon snapping away, getting a number of fine views, while with Jack at the wheel, and Dick to watch the motor, the Tag swept slowly into the river. The current was not strong at this point, and it was possible to slow down to half speed, as the lads did not know the character of the water, nor how much depth there was, though the Tag did not draw more than two feet. “Let’s see who’ll spot the old mill first,” proposed Tom, as he adjusted his camera to take more pictures when the ruin should be sighted. “I’d rather get the first sight of the hidden treasure,” declared Jack, who seemed to have more faith in the existence of the secret horde than did the others. “Anyone can see a mill,” he went on, “but it takes an eagle eye to spot treasure.” “And I suppose you think you’ve got the eagle eye!” laughed Bert. “Sure I have. Say, Dick, isn’t it almost lunch time?” “I don’t know. I’m not the cook this week. It’s up to Tom.” “Can we eat, Tom?” asked his roommate at Elmwood Hall. “Not until we get to the mill. Work before pleasure, my boy. That’s the rule here.” “Well then, get ready with the grub,” said Jack, quietly, “for there’s your mill,” and he pointed just ahead of them.
  • 69. “By Jove! So it is!” exclaimed Tom. They had gone around a turn in the river, and on one bank, situated on a little rise, were the ruins of an old stone mill. In its day it had been a big structure, built of field stone, and it must have been a substantial place to which the settlers for miles around probably came with their grain. But now it was in ruins, through the ravages of time and the hands of those who sought the treasure. As the boat approached it the boys could see where a flume had been built to take the water from the river, and direct it over a big wheel. Of the latter there was little left. Trees and underbrush grew up close to the old structure, near which were the rotting remains of a wharf where, in the olden days, likely, the craft of the settlers had tied up when they came with grist. “Say, it’s a wonderful ruin all right,” said Tom in a low voice. “Put over to shore, Jack, while I get a picture. Then we’ll get out and have a look around.” As Tom focused his camera, and clicked the shutter, there was a movement in the tangle of vines and bushes near what had been the main entrance to the mill. “Look out!” exclaimed Jack. “Some one’s coming!”
  • 70. CHAPTER X A CURIOUS CONFERENCE Holding themselves in readiness for whatever they might see, or for whatever might happen, the boys peered anxiously toward the place whence the noise and movement came. “False alarm!” laughed Tom, as a fox leaped into view and then, seeing human enemies, slunk out of sight. “It made noise enough for a man,” declared Jack. “I sure thought it was the hermit getting ready to repel boarders.” “And treasure seekers,” added Dick. “Well, let’s go ashore,” suggested Bert. “That is, if Tom is done taking fancy snap shots of the old ruin.” “Sure, I’ve got pictures enough for now, though I may want some from the other side,” assented our hero. Making the boat fast to the rotting wharf, the four lads climbed out and made ready to inspect the old ruin. “Look out!” suddenly called Tom. “That’s a weak plank you’re stepping on, Jack. You’ll be through it in another minute!” He made a grab for his chum, but it was too late. Jack, who had hurried on in advance of the others, had stepped on a board of the wharf that was but a mere rotten shell, and, an instant later, one foot went through it, and Jack slipped down to his hip, the other leg doubled up under him. “Help! Help!” he cried, in mock seriousness. “One foot’s in the water, and the other will be in a minute.”
  • 71. “Are you hurt?” asked Bert anxiously. “No, but if this leg isn’t skinned all the way up I’m a loon. Pull me out, can’t you?” As Bert and Dick started toward him Tom called: “Stand back! If we all crowd up on those old boards we will all be through. Wait until I can lay another plank down, that isn’t so near gone. Then we can give you a hand.” With the aid of Bert and Dick, our hero ripped off a more substantial board, and then, stepping on this they managed to pull Jack from his uncomfortable position, for he could not help himself. “Well, how about you?” asked Tom, when they had all made their way off the old wharf to shore. “Oh, so-so. I’m badly battered up, but still in the ring. One foot is well soaked, but it’s warm weather and I guess I won’t get the epizootic. Say, though, I’m going to be lame,” and Jack limped along. An examination showed that his right leg was painfully skinned and bruised, where it had scraped on the edges of the hole in the plank, as his foot went through the timber. “We’ll bandage it up when we get to camp,” said Tom, as he used an extra handkerchief on the worst cut of his chum’s leg. “Do you feel able to go on to the mill, or shall we turn back, Jack?” “Go on, of course,” declared the injured one. “I’m not going to let a little thing like a game leg stand between me and a treasure hunt. Lead on, captain!” “That’s the talk!” exclaimed Bert. “You’ll get the best of the pirates’ hoard yet.” “Now go a bit easy,” cautioned Tom. “It may be that Old Wallace is around somewhere, and, as this is his property, he’d be justified in making a row if he found us here. So go a bit slow until we size up the situation.”
  • 72. They were on the lower side of the mill now, the side nearest the river. The ancient structure consisted of three stories. The lower one was a sort of basement, on a level with the lower ground, where it was evident that wagons had driven in to receive their loads of grain. Here too, was some of the old machinery of the mill, the levers that controlled the water gate and other things, but now all rotted and fallen into decay. “Say, this would be the place where the treasure would be buried, if anywhere,” declared Jack. “I don’t think so,” spoke Tom. “It’s too conspicuous.” “That’s just it,” argued Jack. “The more conspicuous a thing is, the harder it is to find it, sometimes. Nothing is more difficult to pick up, sometimes, than something right under your nose, as the saying is.” “That’s right,” agreed Bert. “Did you ever play the geography game?” “No. What is it?” asked Tom. “Well, you take a big map, and ask a person to find some country, city, lake or river, as the case is. Most persons pick out for the puzzle a name printed in very small type, but those who know select a name printed in big letters, that take up half the map, maybe. And it most always happens that this is the hardest to find. I didn’t originate that,” he added, modestly. “I think Poe speaks of it in one of his stories.” “That’s right,” agreed Tom. “At any rate some one has had a try for the treasure here, at any rate, if signs of digging go for anything.” This was indeed so, for the ground was torn up, and in many places stones had been knocked out of the thick walls, as if some one had looked for secret hiding places. “Well, we can’t stop to dig now,” said Tom. “But if things go right we may later. Let’s go up on the main floor,” and he started toward an ancient doorway.
  • 73. “Not there!” cried Jack, holding back his chum. “Why not?” “The boards there will be as rotten as those on the wharf, and we’ll all take a tumble. Let’s go outside and around on the solid earth. I don’t want to put my other leg out of commission,” and he limped out of the basement of the ancient mill. The others followed, and soon they stood in the doorway of what had evidently been the main entrance to the ancient structure. It was on a level with the higher ground, farther back from the river. This floor contained the mill-stones, now fallen from their position, and encumbered with wreckage. There were several rooms, opening one into the other, now that the doors had fallen from their hinges, and here were holes that went through to the floor above. These holes had once contained the chutes through which the grain was fed to the mill-stones. “There might be treasure here almost anywhere,” remarked Jack, as he looked about. “And it’s been pretty well grubbed for,” commented Tom. “They’ve almost ripped the insides out of the mill looking for it. I suppose old Wallace has cut and sawed and pulled apart until it’s a wonder the old mill hangs together.” “It’s a well-built old place,” said Dick. “The stone walls are thick. There may be a hiding place in them.” “I shouldn’t wonder,” and Bert shrugged his shoulders. “Well, it’s going to be a job to take them apart all right,” and he looked at the stones imbedded in mortar that was as good still as it was the day it was mixed. The boys wandered about the main floor, and looked for a place to ascend to the third story, but there seemed to be none. “If we had a rope we could make it,” said Tom. “We’ll bring one next time.”
  • 74. “Huh! How you going to get up there to fasten it?” asked Bert. “Tie a stick on the end, throw the stick up, and when it catches, crossways, in one of the chute-holes we can go up easily enough.” “Good boy! Bright idea!” complimented Jack. “Well, let’s see if we can find where old Wallace hangs out. We haven’t come across his living quarters yet.” There were several rooms they had not yet explored, and they now proceeded to visit them. They had evidently been the living apartments of the former miller, but now they were pretty much in ruins. “No signs of a course dinner having been prepared here,” commented Tom. “It smells as musty as time. He must hang out somewhere else.” “Upstairs, I’ll wager,” said Dick. “But how does he get up?” asked Jack. “Oh, he has some secret way,” declared Tom. “We’ll have to get a rope and explore that third story all right.” “Say, maybe we’re staying too long now,” suggested Bert. “Old Wallace may come along and nab us. We’ve seen all there is to, I guess, except upstairs.” “But we haven’t seen any gold,” said Jack. “I want to find some before I go back.” “Get out!” laughed Tom. “All the gold there is in this mill you can put in your eye. But I think it might be a good idea to look outside a bit. Maybe there’s some outbuilding, or some secret cache where the pirates or settlers hid their stuff. We’ll take a look.” “And then we’ll have some eats!” suggested Jack. “I’m as hungry as the proverbial bear.” They strolled about the old mill, and saw more signs of where a search had been made for the reputed treasure. Holes innumerable
  • 75. were on every side, but the attempts to locate the hidden gold had soon been given over, for the excavations were shallow. “Now for the eats!” exclaimed Jack, as they started for the dock where their boat was tied. “Lands! but I’m stiff!” He really was limping painfully, and his chums had to help him down the hill to the river. As they approached their boat Tom, who was slightly in advance, uttered an exclamation of surprise as he peered along a path that led up the river. “What is it?” asked Dick. “Look,” was the answer. “Old Wallace! We got away just in time.” “And see who’s with him!” exclaimed Jack, in a hoarse whisper. “Professor Skeel!” “By Jove! So it is!” gasped Tom. “Wonders will never cease. Have they seen us?” It was evident they had not, but to make sure of it the boys hurried behind a screen of bushes, where they could see but not be observed. “Look!” exclaimed Tom again. “They’re going to have a conference.” As he spoke the others could see that the former professor and the old hermit had come to a halt in a place where the path widened. It was in a little glade, and, sitting down beneath the trees, the two men, one of whom had played such a strange part in Tom’s life, and the other, who was destined to, proceeded to talk earnestly. What they said could not be heard, but it was evident that it was some subject that interested them both, for they held their heads close together as if afraid of being overheard. They little realized that they were being watched. “What are they doing now?” asked Dick. “The old hermit has some sort of a paper,” said Tom.
  • 76. “And he’s showing it to Mr. Skeel,” added Bert. “Maybe it’s some sort of map to tell where the treasure is,” suggested Dick. “But why would he be showing it to our old professor?” asked Tom. “If he wants to keep it a secret why is he giving it away like that?” “Hard to say,” commented Jack. “I think, though——” He did not finish, for at that moment Mr. Skeel and the hermit leaped to their feet and gazed down the path as though they heard some one coming.
  • 77. CHAPTER XI AN ANGRY HERMIT “Something new on the programme,” commented Tom in low voice. “Do you think they heard us?” asked Dick. “No, they couldn’t. And they don’t see us. They’re looking the other way,” said Jack. “But there’s something doing,” declared Bert. “I wonder what it is?” They had their answer a moment later, when there came into view around the bend in the path Sam Heller and Nick Johnson. “Well, I’ll be jiggered!” gasped Tom. “We meet them at every turn.” “I wonder how they got here so soon after we met them?” asked Bert. “It’s quite a distance to walk.” “Maybe they took a short cut,” suggested Jack. “Hush! Look what’s going on now,” advised Tom. As they glanced toward where the professor and the hermit had held a conference, they saw the old man transported into one of his fits of rage. He stamped about, and shook his fist at Sam and Nick, occasionally changing by making threatening gestures at Mr. Skeel. “Say, he’s the limit!” murmured Dick. “Listen,” cautioned Tom. “He’s saying something.”
  • 78. “Leave here! Leave here at once!” commanded old Wallace, almost hitting the two lads as he shook his fists at them. “How dare you come on my property? You are after the treasure; are you? Well, you shall never find it! I will locate it! I will make the old mill give up its secret! Be off!” “Wait, wait,” said Mr. Skeel in a calm voice, laying his hand on the hermit’s arm. “Ha! You too are in a plot against me, I believe!” cried the angry hermit. “I am sorry I ever had anything to do with you. Go away!” and he took hold of the professor, and began shoving him away down the path. “One minute,” said Mr. Skeel in soothing tones, much different from the harsh ones he had almost constantly used in his classes at Elmwood Hall. “What is it you object to?” “These lads—what are they doing here? Are they spying on me?” and the aged man pointed at Nick and Sam. “They are my assistants,” said the professor soothingly, and, though he spoke in a low tone, Tom and his chums could hear him. “Without their aid I can not help you,” Mr. Skeel went on, and when the hermit’s back was turned toward him our hidden friends distinctly saw the professor make a signal of caution and of acquiescence toward the two lads, who craftily nodded their understanding. “Your assistants?” asked the hermit. “Yes. If you want me to help you I must have them to help me. I would have told you about them, but I did not get the chance until they came so unexpectedly. Had they known that you objected to their presence they would have remained away. But I assure you that you can trust them.” “Well,” said the hermit, bitterly, “since I have told you part of my secret, and trusted you with it, I suppose your assistants must be in on it. But no more! No more!” and he shook his fist toward the
  • 79. clouds, and glanced around as though he feared more intruders. “There were some other boys around the other day,” the aged man went on, “and if I find them sneaking about my mill it will be the worse for them.” “Say, we did get away just in time,” whispered Jack. “That’s right,” agreed Dick. “But what in the world does Skeel mean by saying he is going to help Wallace, and that Sam and Nick are his assistants, I wonder?” asked Bert. “That’s easy to guess,” answered Tom. “Skeel, somehow or other, has heard about the treasure. Now he’s trying to soft-soap the hermit into letting him have a hunt for it. Probably he’s promised to turn most of it over to the old man.” “I think I see him doing it, if he finds it,” commented Bert. “And Skeel has the nerve to say that Sam and Nick are his helpers,” said Jack. “Hot helpers they are!” “Oh, that was just a bit of jollying, thought up on the spur of the moment,” declared Tom. “He didn’t figure on Sam and Nick following him, and he had to concoct some story to account for their presence. Though I don’t doubt but what Skeel, and those two cronies, are in thick about some scheme.” “Searching for the treasure?” asked Dick. “I believe so. Well, they’ve got one advantage of us, but maybe we can get ahead of them yet,” spoke Tom. “If only we can get a chance to do some exploring we’ll do it. But we can’t do anything more now.” “No, let’s go down to the boat and eat,” suggested Jack. “I’m still hungry.” “Wait a minute,” advised Tom. “I think they’re going to move on, and we don’t want to run into them.”
  • 80. As they watched they saw Sam and Nick turn and retrace their steps back along the path. They had held a little conversation with Mr. Skeel, to one side, so that the hermit had not heard, though he eyed them suspiciously. Then Mr. Skeel and the old man resumed their talk. “Lucky that Sam and Nick didn’t come this way,” said Tom, as he helped Jack to stand up. “Now don’t make any more noise than you can help, or they may hear us.” “They’ll hear the boat when it starts,” said Dick. “I’ll drift down the river a bit before I crank up,” spoke Tom. “Come on, everybody.” They started down the bank, toward their boat, having come to a halt a little distance from it. Suddenly Dick, who was in the rear, uttered an exclamation. “What is it?” called Tom sharply. “They’re going away—Skeel and the hermit, and one of them has dropped a piece of paper on the path.” “A piece of paper!” exclaimed Tom. “We must have that! Here, wait a minute! If they don’t miss it, and come back for it, I’ll get it.” He crawled cautiously back to his former post of observation behind the screen of bushes.
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