The document discusses emulsions, particularly microemulsions and multiple emulsions, detailing their definitions, types, advantages, and theories of formation. It emphasizes the stability issues related to emulsions, including flocculation, creaming, coalescence, breaking, and phase inversion, as well as methods for preserving emulsions and preventing microbial growth. Additionally, the rheological properties of emulsions are evaluated for various applications in drug delivery and stability studies.

1. Emulsions
COARSE DISPERSION
Nabeela Moosakutty
Asst. Professor
Dept.of Pharmaceutics
KTN College of Pharmacy

2. MICROEMULSIONS
Definition
These are homogenous, transparent and
thermodynamically stable dispersion of water and oil
stabilized by surfactant and co-surfactants
Consists of globules less than 0.1 μm in diameter
Types
1. Oil dispersed in water (o/w) - oil fraction low
2. Water dispersed in oil (w/o) - water fraction low
3. Bicontinuous (amount of oil and water are same)
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3. 3
Advantages
Thermodynamically stable, long shelf life
Potential reservoir of lipophilic or hydrophilic drug
Enhance the absorption and permeation of drugs through biological membranes
Increased solubility and stability of drugs
Ease and economical scale-up
Greater effect at lower concentration
Enhances the bioavailability of poorly soluble drugs
Theories of microemulsion
1. Interfacial or mixed film theory
Microemulsions are formed spontaneously due to formation of complex film at the interface by a mixture of surfactant
and co-surfactant, As a result of which the interfacial tension reduces
2. Solubilization theory
Microemulsions are considered to be thermodynamically stable solutions of water swollen (w/o) or oil swollen (o/w)
spherical micelles
3. Thermodynamic theory
The free energy of microemulsion formation is dependent on the role of surfactant in lowering the surface tension at the
interface and increasing the entropy of the system

4. MULTIPLE EMULSIONS
▸ Multiple emulsions are complex polydispersed systems where both oil in water and water in oil
emulsion exists simultaneously which are stabilized by lipophilic and hydrophilic surfactants
respectively
▸ The ratio of these surfactants is important in achieving stable multiple emulsions
▸ They are also known as “Double emulsion” or “emulsion-within-emulsion”
Types
Oil-in-water-in-oil (O/W/O)
An o/w emulsion is dispersed in an oil continuous phase
Water-in-oil-in-water (W/O/W)
a w/o emulsion is dispersed in a water-continuous phase
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5. THEORIES OF EMULSIFICATION
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1. MONOMOLECULAR ADSORPTION THEORY
2. MULTIMOLECULAR ADSORPTION THEORY
3. SOLID PARTICLE ADSORPTION THEORY
4. ELECTRICAL DOUBLE LAYER THEORY
5. ORIENTED WEDGE THEORY

6. 6
MONOMOLECULAR ADSORPTION THEORY
● Surfactants adsorb at the oil-water interface and form a
monomolecular film
This film rapidly envelopes the droplets
● They are very compact, elastic, flexible, strong and cannot be
easily broken
● For getting better stable emulsions combination of surfactants
[surfactant blend] are used rather than a single one
● The surfactant blend consists of both water soluble and oil
soluble surfactants in order to approach the interface from
aqueous and oil phase sides
● At interface the surfactant blend interact to form a complex and
condense a monomolecular film
● Ex: A combination of Sodium cetyl sulfate (hydrophilic) and
Cholesterol (lipophilic) forms a close packed complex film at
the interface that produces an excellent emulsion
● Surfactant blend also produces poor quality emulsions if the
interaction between them is not strong enough at the interface

7. 7
MULTIMOLECULAR ADSORPTION THEORY
● The emulsifying agents tend to form a multimolecular film around the globules and prevent coalescence
● They also reduces interfacial tension moderately
● Ex: Acacia and Gelatin
SOLID PARTICLE ADSORPTION THEORY
● The finely divided solid particles adsorb
at the oil-water interface and form a rigid
film of closely packed solids
● This film acts as a mechanical barrier and
prevents the coalescence of globules
● Depending on the affinity of the
emulsifier to a particular phase we can
both types of emulsions
Ex: Bentonite - o/w and w/o
Veegum - o/w

8. 8
ELECTRICAL DOUBLE LAYER THEORY
The oil droplets contain either negative or
positive charge
If the oil droplet carries negative charge then it
will adsorb positive ions from the solution
Formation of stern layer (immovable) and
diffuse layer (movable)
Formation of zeta potential at slipping line
(boundary line of diffuse layer)
Due to zeta potential oil droplets produces
repulsion forces
Electrical barrier is created between oil
droplets
Oil droplets remain in dispersed phase thereby
prevent coalescence and breaking of emulsion

9. 9
ORIENTED WEDGE THEORY
This theory deals with formation of monomolecular layers of emulsifying agent curved around a droplet of the
internal phase of the emulsion
In a system containing 2 immiscible liquids, Emulsifying agent would be preferentially soluble in one of the phases
and embedded in that phase
That means an emulsifying agent having a greater hydrophilic character will promote o/w emulsion and vice-versa
Ex: Sodium oleate is dispersed in water and not oil, It forms a film which is wetted by water than by oil. This leads
the film to curve so that it encloses globules of oil in water
A: Emulsifier molecule oriented at interface. Dotted line indicates large volume occupied by polar head due to formation of hydrated complex
B: Close packing of molecules fits this curvature

10. PHYSICAL STABILITY OF EMULSIONS
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1. FLOCCULATION
2. CREAMING
3. COALESCENCE
4. BREAKING
5. PHASE INVERSION
Emulsion stability can be defined as the system’s ability to resist changes in its physicochemical properties
over time
Emulsion Instability Problems

11. 11
Flocculation
● Flocculation is defined as the association of globules within an emulsion to form large
aggregates and redispersible upon shaking
● Flocculation is due to the interaction of attractive and repulsive forces
● The extent of flocculation of globules depends on
Globule size distribution (can be prevented by achieving uniform sized globules through proper size
reduction process)
Charge on the globule surface (exerts repulsive forces - prevented by addition of electrolytes
or addition of ionic emulsifying agents)
Viscosity of the external medium (can be prevented by adding viscosity improving
agents)

12. 12
Creaming
● Flocs or individual globules slowly move either upward or downward leading to creaming
● It is the concentration of globules at the top or bottom region of the emulsion
● It is due to density differences in the two phases
● It is a reversible process cream can be easily redispersible upon agitation
● Two Types
Upward creaming - due to less denser internal phase
Downward creaming - due to heavier internal phase and gravitational pull
● Creaming is influenced by
Globule size (can be prevented by reducing particle size by homogenization)
Viscosity of the dispersion medium (can be prevented by addition of thickening agents such as tragacanth,
sodium alginate)
Density difference of the two phases (Reducing the density difference between the two phases by the use of
density modifiers) - In general the density of aq. phase is higher than the oil phase then add oil soluble
substances such as bromoform, carbon tetrachloride etc

13. 13
Coalescence
● A few globules tend to fuse with each other and form bigger globules
● The emulsifier film around the globules is destroyed to some extent
● This step is characterized by increased globule size and reduced number of globules
● Coalescence is an irreversible process because the film that surrounds the individual globules is destroyed
● It is observed due to
Insufficient amount of the emulsifying agent
Altered partitioning of the emulsifying agent
Incompatibilities among emulsifying agents
● Can be prevented by
Phase volume ratio
● Represents the relative volume of water to oil in an emulsion
● At higher ratio (>74% of oil to water), globules are closely packed and small globules occupy the void spaces
between the bigger globules
● Globules gets compressed leads to fusion of adjacent globules
● The dispersed phase should be less than 74%
● A Phase volume ratio of 50/50 is likely to give most stable emulsions

14. 14
Breaking
● Indicated by complete separation of oil and aqueous phases
● It is an irreversible process
● The protective sheath around the globules is completely destroyed
● Can be prevented by
Uniformity of particle size of dispersed phase
Increase in viscosity of the emulsion
Phase volume ratio
Phase Inversion
● This involves change of emulsion type from o/w to w/o or vice versa
● Phase inversion can occur by the addition of an electrolyte or by changing the phase volume ratio
● Addition of monovalent cations promotes the formation of o/w emulsions, whereas the addition of divalent cations
increases the propensity toward the formation of w/o emulsions
● Ex: an o/w emulsion stabilized with sodium stearate can be inverted to a w/o emulsion by adding calcium chloride to
form calcium stearate
● Can be prevented by choosing proper emulsifying agent in suitable conc. and Phase volume ratio
Breaking and Phase Inversion

15. PRESERVATION OF EMULSIONS
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Some ingredients of emulsions promote growth of microorganisms by providing nutrients
Bacteria feed on non-ionic and ionic surfactants, glycerin, and Emulsifying agents (natural polysaccharides) cause
deterioration of emulsions
Ex: arachis oil promote growth of aspergillus, Rhizopus
while liquid paraffin promote growth of some spp. Brucitisses
For these reasons, emulsions should be formulated with a preservative
Contamination of emulsions could be due to:
· Contaminated Emulgent
· Deionized water (Bacteria grow in resin beds)
· Equipment
· Poorly closed container
Equation used to calculate the conc. of preservative

16. 16
Emulsion should be free from microbial contamination and growth
In case of parenteral emulsions sterility of the product is essential
Microbial contamination may cause a change in organoleptic properties of emulsion, gas production, hydrolysis, pH
change and breaking of emulsion
Commonly used preservatives: Ascorbic acid, Benzoic acid, Sodium benzoate, cetrimide, chloroform, phenoxyethanol,
cresol derivatives such as chlorocresol, Parahydroxybenzoate esters such as methyl paraben, propyl and butyl parabens
etc
The factors which is to be considered during selection of preservatives are:
Type of emulsion
Type of container
Volume fraction of aq.phase
Degree of aeration
Nutritive value
pH of aq.phase
Binding of ingredients in the formulation

17. 17
An ideal preservative should be
Non-irritant
Non-toxic in the conc. used
Tasteless, colourless and odourless
Stable and effective over a wide range of pH and temperature
Have bactericidal properties
Unionized (so as to penetrate the bacteria membrane)
Physically and chemically compatible with other ingredients of the emulsions and with the packaging container
Precautions should be taken to prevent microbial growth
1. Use of uncontaminated raw material
2. Proper cleaning of equipments
3. Sterilization of equipments either by steam or by hot air oven
Preservation from oxidation
The oxygen present in the atmosphere cause oxidative changes such as rancidity and spoilage
Can be prevented by the use of antioxidants
Ex: Alkyl gallates, BHA (Butylated hydroxyanisole), BHT (Butylated hydroxytoluene)

18. RHEOLOGICAL PROPERTIES OF EMULSION
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● Emulsions are evaluated for its flow behaviour. The flow behaviour are desirable for
Removal of an emulsion from a bottle
Flow of emulsion through a hypodermic needle
Spreadability of an emulsion on the skin
Stress induced flow changes during manufacture
Drug release
Stability studies
● Dilute emulsions exhibit “Newtonian flow” while concentrated emulsions exhibit “non-Newtonian flow”
● The increase in the viscosity of the emulsion reduce the flocculation of globules. i.e; an optimum
viscosity is desirable for good stability
● Multipoint viscometers such as Cone and Plate viscometer or Cup and Bob viscometer can be
employed for evaluation

19. 19
● The rheological property of emulsion can be controlled by
Conc. of dispersed phase
Particle size of dispersed phase
Viscosity of continuous phase
Nature and conc. of emulsifying system
● The type of flow can be obtained based on Phase volume ratio
Phase volume ratio Type of flow
5% Newtonian
50% Pseudoplastic
74% Plastic flow or Phase inversion