Theories of dispersion

THEORIES OF
DISPERSION
PRESENTED BY
RAHUL KRISHNAN.P.R
DEPT.OF PHARMACEUTICS
GRACE COLLEGE OF PHARMACY, PALAKKAD
1

CONTENTS
1.PHARMACEUTICAL DISPERSIONS-
INTRODUCTION
(EMULSIONSUSPENSION,COLLOIDS)
2.THEORIES OF DISPERSION
3.LIMITATIONS OF THEORIES
4.MODERN APPROACH TO THEORIES OF
DISPERSION
2

PHARMACEUTICAL DISPERSIONS
 Dispersed systems consist of particulate matter, known
as the dispersed phase, distributed throughout a
continuous or dispersion medium.
 The dispersed material may range in size from particles
of atomic and molecular dimensions to particles whose
size is measured in millimeters.
3

COARSE DISPERSION SYSTEMS
A. Emulsion
B. Colloids
C. Suspension
4

EMULSIONS
 An emulsion is a biphasic liquid preparation containing
two immiscible liquids, one of which is dispersed as
minute globules into the other.
 The liquid which is converted into minute globules is
called the ‘dispersed phase’ and the liquid in which the
globules are dispersed is called the ‘continuous phase’.
 Normally, two immiscible liquids cannot be dispersed for
a long period.
 So, an emulsifying agent is added to the system.
 It forms a film around the globules in order to scatter
them indefinitely in the continuous phase, so that a stable
emulsion is formed.
 The globule size in emulsion varies from 0.25 to 25 µm.
5

SUSPENSION
 Suspension are the biphasic liquid dosage form of
medicament in which the finely divided solid particles
ranging from 0.5 to 5.0 micron are dispersed in a liquid
or semisolid vehicle.
 The solid particles act as disperse phase whereas liquid
vehicle acts as the continuous phase.
 Suspension are generally taken orally or by parenteral
route.
 They are also used for external applications.
6

THEORIES OF DISPERSION
 There are mainly 4 theories:
 1.Viscousity theory
 Film Theory or adsorption theory
 Wedge Theory
 Interfacial tension theory
7

1.VISCOUSITY THEORY
 As per this theory, an increase in viscosity of
emulsion will lead to an increase in stability.
 LIMITATIONS OF THE THEORY
 This theory failed to explain about the milk,
which shows considerable stability even though
its viscosity is less.
8

2.FILM THEORY OR ADSORPTION
THEORY
 As per this theory, the added emulsifying agent
forms a mechanical film by getting adsorbed at
the interface of the liquids (i.e at the interface
between the dispersed globules and the
dispersion medium)
 This offers stability to the emulsion
LIMITATIONS
 This theory could not explain the formation of
type of emulsion
9

3.WEDGE THEORY
 According to this theory, monovalent soaps like
sodium sterate gives o/w type emulsion and
divalent soaps like calcium sterate gives w/o type
emulsion.
 This was explained by the successful
accommodation of the soap molecules at the
interface and subsequent possible orientation of the
soap molecules to give the type of emulsion
11

HOW MONOVALENT SOAPS STABILIZES
EMULSION
12

The polar head will be
attracted towards the
water, and non polar
tail will be attracted
towards the oil,
allowing water and oil
to combine
13

 This is how divalent soaps are oriented to for a W/O
type emulsion.
14

LIMITATIONS
 This theory could not explain the stability of an
emulsion.
 The calcium sterate will not obey this theory, it will
ionize and will not exist as a wedge.
15

4.INTERFACIAL TENSION THEORY
 Initially when the oil and water are mixed together,
they will become immiscible due to the interfacial
tension.
 The added emulsifying agent reduces the interfacial
tension between the oil and the water phase.
 Hence a stable emulsion is formed.
16

MODERN APPROACH
 When a liquid is shaken with another immiscible
liquid, the liquid in small quantities disperse as fine
droplets in another liquid which is in large quantity.
 Depending upon the agitation rate or the shear
used, the size of the globules vary.
 Higher shear rates produces smaller globules.
 The time of agitation is also important. Mean size of
the droplets decreases rapidly in first few seconds.
 Limiting size range reaches within 1 to 5 minutes.
18

 Such a dispersion is thermodynamically instable
and the two liquids separates into two clearly
defined layers which are thermodynamically stable.
 This is because, the cohesive forces between the
molecules of each liquid is greater than adhesive
forces between the molecules of two liquids.
 The existence of an emulsion is the result of two
competing forces:
1.The dispersion of liquid in the form of droplets
throughout the other
2. The combination of these droplets to reform
initial bulk liquid.
19

 When a liquid is broken into small particles, the
interfacial area of the globules will be enormous as
compared to surface area of the orginal liquid.
EXAMPLE:
 If 1cm3 of mineral oil is dispersed into globules of
a volume of 0.01 microns, into 1 cm3 of water, the
surface area of oil droplets becomes 600sq.meters.
 But the total volume of the system remains same.
 Hence the system becomes thermodynamically
unstable.
 Thus the emulsifying agents becomes necessary.
20

 Based on the nature of the films formed at the
interface, mechanism of action of the emulsifying
agents have been divided into 3:
1.MonomolecularAdsorption and film formation
2.Multimolecular adsorption
3. Solid particle adsorption.
21

MONOMOLECULAR ADSORPTION AND
FILM FORMATION
 Surfactants adsorb at the oil-water interface and
forms a monomolecular film.
 This film rapidly envelops the droplets as soon as
they are formed.
 Agents having higher interfacial activity are better
suited for this purpose.
 The monomolecular film should be compact and
strong enough so that it cannot be easily broken.
 If it is broken, it should be elastic and flexible
enough, so that it can be reformed on rapid
agitation.
22

 Combination of surfactants are used rather than a
single one
NB:
(a)Combination of sodium cetyl sulfate and oleyl
alcohol do not form a close packed film, and
therefore they form poor emulsions
(b)Cetyl alcohol and sodium oleate produce a close
packed film, but complexation is negligible, and
hence produce poor emulsions.
23

MULTIMOLECULAR ADSORPTION
 The emulsifying agents such as acacia and gelatin,
tend to form a multimolecular film around the
globules and prevents the instability, mainly
coalescence.
 They are effective at high concentrations, and
promote the formation of o/w type emulsion owing
to their hydrophobicity.
25

SOLID PARTICLE ADSORPTION
 The finely divided solid particles adsorb at the oil-
water interface and forms a rigid film of closely
packed solids.
 This film act as a mechanical barrier and prevents
coalescence of globules.
 These tend to produce coarse emulsions.
 Depending upon the affinity of the emulsifie to a
particular phase, one can prepare both types of
emulsions; e.g.:
1.Bentonite-o/w and w/o
2.Veegum-o/w 26

 Diagrammatic representation of 3 mechanisms of
emulsion formation.
27

REFERENCE
 Remington, vol.1 page no: 264-272
 Physical Pharmaceutics by Manavalan
Ramaswami, Page no: 248-252
 Physical Pharmaceutics by CVS Subramanyam,
Page No:403-407
28

Theories of dispersion

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