Module 1 e (theory of disperse system).pptx
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The document discusses key concepts related to disperse systems, including: - Particle properties like shape and size distribution impact properties like viscosity, packing, and stability. - Surface charge and interfacial phenomena like van der Waals forces, electrical double layers, and zeta potential determine whether attractive or repulsive forces dominate between particles. - The DLVO theory describes how the total interaction potential (VT) is determined by a balance of attractive van der Waals forces (VA) and repulsive electrostatic forces (VR), with stability requiring the maximum repulsion (Vmax) to exceed ~50 mV. - Systems can be stabilized against instability through electrostatic or steric repulsion between particles.
Module 1 e (theory of disperse system).pptx
- 2. Comparison Reference : Martin’s physical pharmacy and pharmaceutical sciences : physical chemical and biopharmaceutical principles in the pharmaceutical sciences.—6th ed. / editor, Patrick J. Sinko ; assistant editor, Yashveer Singh.
- 4. Fundamental Properties Particle properties Surface and interfacial phenomena Instability and stabilization Rheological properties
- 5. Particle properties Shape Particle size and size distribution Shape: Important for understanding behavior of suspensions during storage Particle shape may have an impact on packing of sediment, thereby on product suspendability and stability. Packing stability is weight to volume ratio of sediment at equilibrium. Particle shape affects viscosity of colloidal dispersions. Spherocolloids-low viscous, Linear – more viscous Changes in particle shape of colloids affects its flow, sedimentation and osmostic pressure
- 6. Particle size and size distribution: Particle size affects absorption Certain types of dosage forms require specific size range Suspensions, aerosols delivering drugs into respiratory tract should contain particles in the order of 1-5 μm and no particles larger than 10 μm Size of a spherical particle expressed in terms of its diameter Asymmetrical particles –equivalent spherical diameter is used to relate size of the particle to the dia of perfect sphere having same surface area or same volume. Particle size distribution: number distribution /weight distribution A wide particle size distribution often results in high density suspension whereas widely differing particle shape results in often produce low density slurries
- 7. Surface Properties & Interfacial phenomena Two important factors : As particle size is reduced, specific surface is increased :Presence of an electrical charge on the particle surface Interface: Defined as a boundary between two phases Surface free energy=Δ𝐺 = 𝑌𝑠𝑙 . ΔA; equilibrium will be reached when Δ𝐺=0 Particle Interactions: Both attractive and repulsive forces exist These forces depend upon: the nature, size and orientation of species distance of separation between and among the particles of dispersed phase and dispersion medium respectively.
- 8. The balance between these forces determines the overall characteristics of the system The particles in a disperse system with a liquid or gas being the dispersion medium are thermally mobile and occasionally collide as a result of Brownian motion . As the particles approach one another,both attractive and repulsive forces are operative If attractive forces prevail, agglomerates results-indicates instability If repulsive forces dominate, a homogeneous dispersedly or stability dispersion remains
- 9. Various types of attractive forces a) Dipole-dipole or Keesome orientation forces b) Dipole-induced dipole or Debye induction forces c) Induced dipole –induced dipole or London dispersion forces d) Electrostatic interactions between charged particles : Strongest force-either attractive or repulsive and are effective over a relatively long range and are dependent on the ionic charge and size Forces that are weaker and effective over a shorter distance include other types of electrostatic forces such as ion- dipole, ion-induced dipole and (a-c). (a-c) forces are referred to as Van der Waals forces. short –range type of interaction, varying inversely with r6
- 10. 10 Electrical properties of interface Particles dispersed in liquid media may become charged mainly in one of two ways. The first involves the selective adsorption of a particular ionic species present in solution. This may be an ion added to the solution or, in the case of pure water, it may be the hydronium or hydroxyl ion. Second, charges on particles arise from ionization of groups (such as COOH) that may be situated at the surface of the particle A third, less common origin for the charge on a particle surface is thought to arise when there is a difference in dielectric constant between the particle and its dispersion medium
- 11. • Some of the cations are adsorbed onto the surface, giving it a positive charge. • Remaining in solution are the rest of the cations plus the total number of anions added. These anions are attracted to the positively charged surface by electric forces that also serve to repel the approach of any further cations once the initial adsorption is complete. • In addition to these electric forces, thermal motion tends to produce an equal distribution of all the ions in solution. • As a result, an equilibrium situation is set up in which some of the excess anions approach the surface, whereas the remainder are distributed in decreasing amounts as one proceeds away from the charged surface. • At a particular distance from the surface, the concentrations of anions and cations are equal, that is, conditions of electric neutrality prevail. 11
- 12. aa′ is the surface of the solid. The adsorbed ions that give the surface its positive charge are referred to as the potential-determining ions. Immediately adjacent to this surface layer is a region of tightly bound solvent molecules, together with some negative ions, also tightly bound to the surface. The limit of this region is given by the line bb′ These ions, having a charge opposite to that of the potential-determining ions, are known as counterions or gegenions. The degree of attraction of the solvent molecules and counterions is such that if the surface is moved relative to the liquid, the shear plane is bb′ rather than aa′, the true surface.
- 13. • In the region bounded by the lines bb′ and cc′, there is an excess of negative ions. • The potential at bb′ is still positive because, there are fewer anions in the tightly bound layer than cations adsorbed onto the surface of the solid. • Beyond cc′, the distribution of ions is uniform and electric neutrality is obtained. • Thus, the electric distribution at the interface is equivalent to a double layer of charge, the first layer (extending from aa′ to bb′) tightly bound and a second layer (from bb′ to cc′) that is more diffuse. The so- called diffuse double layer therefore extends from aa′ to cc′ 13
- 14. • The potential at the solid surface aa′ due to the potential-determining ion and is defined as the difference in potential between the actual surface and the electroneutral region of the solution. • The potential located at the shear plane bb′ is known as the electrokinetic, or zeta, potential, ζ. • The zeta potential is defined as the difference in potential between the surface of the tightly bound layer (shear plane) and the electroneutral region of the solution electrothermodynamic (Nernst) potential, E. 14
- 15. DLVO theory It relates the stability of a disperse system to the electrolyte content in the continuous phase and provides an insight into the factors responsible for controlling the rate at which particles in disperse systems come contact or aggregate. DLVO theory applicable to colloids, suspensions, and O/W emulsions. VT = VA + VR
- 16. Attractive forces predominant at short distances & the net interaction is attraction in the deep potential energy minimum At greater distances, the electrostatic repulsion energy falls off more rapidly with increasing separation distance than the van der Waals attraction energy and the net interaction is attraction in the shallow secondary minima At intermediate distances, the electrostatic repulsion predominates and net interaction is repulsion with max potential Vmax Stability of a disperse system is indicated by the height of the maximum in the potential energy curve Vmax Value of Vmax necessary to prevent irreversible contact of particles is around 50 mV
- 17. Instability and Stabilization concepts Instability: due to reduced surface energy Disperse system is stable as long as repulsive forces are sufficiently strong to outweigh van der Waals or other attractive forces Repulsive forces can either be: Electrostatic or Steric