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Dodecane droplets in a

continuous phase ofwater/glycerol mixture.

Sodas: Oil in Water emulsion

Milk: Oil in Water

emulsion

Balm: Water in oil emulsion

Mayonnaise: Oil in

Water emulsion

Emulsions

Emulsion

suitable for

intravenous

injection.

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EmulsionDispersion of liquid droplets (dispersed phase) of

certain size within a second immiscible liquid (continuous

phase).

Classification of emulsions

- Based on dispersed phase

Oil in Water (O/W): Oil droplets dispersed in waterWater in Oil (W/O): Water droplets dispersed in oil

Water in Oil in water (W/O/W): Water in Oil emulsion

dispersed in watermultiple emulsion- Based on size of liquid droplets

0.250 mm Macroemulsions

0.010.2 mm Microemulsions

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Advantages

Administration of Distasteful oil, mask the

unpleasant taste

Better and faster absorption

Less irritation to the skin

Sustained release medication

Nutritional supplement

Diagnostic purposes

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Metal cutting oils Margarine Ice cream

Pesticide Asphalt Skin cream

Emulsions encountered in everyday life!

Stability of emulsions may be engineered to vary from

seconds to years depending on application

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Stable dispersions of liquids constituting the dispersed phase,

in an immiscible liquid constituting the continuous phase isbrought about using emulsifying agents such as

Carbohydrates: acacia, tragacanth, agar, chondrus and pectin

Proteins: gelatin, egg yolk and casein

High mol wt alcohols: stearyl alcohol, cetyl alcohol,

glycery monostearate, cholesterolw/o stabilisers

Surfactants: SPAN, TWEEN, organic soaps

( triethanolamine oleate),

Non ionic- pH 3-10, cationic3-7, anionic- greater than 8

Emulsifying Agents

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Surfactants

Anionic Sodium stearate, Potassium laurateSodium dodecyl sulfate, Sodium sulfosuccinate

Nonionic Polyglycol, Fatty acid esters, Lecithin

Cationic Quaternary ammonium salts,

Finely divided Solids

Finely divided solids with amphiphilic properties such as

silica and clay, may also act as emulsifying agents

Others: bentonite, magnesium hydroxide, Al(OH)3

Common Emulsifying Agents

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Based on the Bancrofts rule, many emulsion properties are

governed by the properties of the continuous phase

1. Dye test

2. Dilution test

3. Electrical conductivity measurements

4. Filter paper test

Tests for Emulsion Type

(W/O or O/W emulsions)

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Thermodynamic instability

G = . A

Increase in the surface free energy =

interfacial tension X increased surface area

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Mechanism of emulsification

Monomolecular theory

Surfactants

Reduce interfacial tension

Forms a protective film around globule Ionic surfactant exert repulsion between

globules

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Mechanism

Multimolecular theory

Hydrocolloids form multimolecular

physical barrier around globules there by

prevent coalescence of oil globules

Acacia, gelatin

Solid particle adsorption theory

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Physical InstabilityCreaming: Concentration of globules at the top or bottom of

emulsion.

Reversible process but leads to breaking

Influenced by: Stokes equation

V = h = d2st(so) g

t 18o

-globule size

-Viscosity of dispersion medium

-Difference in the densities of dispersed and dispersion

medium

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Droplets larger than 1 mm may settle preferentially to the top or thebottom under gravitational forces.

Creaming is an instability but not as serious as coalescence or

breaking of emulsion

Probability of creaming can be reduced if

a - droplet radius, - density difference,

g - gravitational constant, H - height of the vessel,

Creaming can be prevented by homogenization. Also by reducing

, creaming may be prevented.

kTgHa 334

Creaming of Emulsions

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Creaming can be

reduced/prevented by Reducing the globule size by

homogenization

Increasing the viscosity of dispersion

medium

Reducing the difference in densities

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Coalescence

Separation of two phases due to fusion of globules.

Also called cracking of emulsion.

Irreversible process.Sheath of EA around globules is lost.

Creaming leads to breaking- globules comes nearer

Breaking of emulsion is observed due to:Insufficient amount of EA

Incompatibility between EA

Alteration in the properties of EA

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O/WW/O

1. The order of addition of the phases

W O + emulsifier W/OO W + emulsifier O/W

2. Nature of emulsifier

Making the emulsifier more oil soluble tends to produce a W/Oemulsion and vice versa.

3. Phase volume ratio

Oil/Water ratioW/O emulsion and viceversa

Inversion of Emulsions (Phase inversion)

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4. Temperature of the system

Temperature of O/W (polyoxyethylenated nonionicsurfactant) makes the emulsifier more hydrophobic and the

emulsion may invert to W/O.

5. Addition of electrolytes and other additives.

Strong electrolytes to O/W (stabilized by ionic surfactants)

may invert to W/O

Example. Inversion of O/W emulsion (stabilized by sodiumcetyl sulfate and cholesterol) to a W/O type upon addition

of polyvalent Ca.

Inversion of Emulsions (Phase inversion)

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Bancroft's rule

Emulsion type depends more on the nature of the emulsifying agentthan on the relative proportions of oil or water present or the

methodology of preparing emulsion.

The phase in which an emulsifier is more soluble constitutes thecontinuous phase

In O/W emulsionsemulsifying agents are more soluble in water

than in oil (High HLB surfactants).

In W/O emulsionsemulsifying agents are more soluble in oil than

in water (Low HLB surfactants).

W/O vs. O/W emulsions

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Rate of coalescencemeasure of emulsion stability.

It depends on:(a) Physical nature of the interfacial surfactant film

For Mechanical stability, surfactant films are characterized

by strong lateral intermolecular forces and high elasticity

Mixed surfactant system preferred over single surfactant.

(Lauryl alcohol + Sodium lauryl sulfate: hydrophobic interactions)

combination of SPAN and TWEEN

Emulsions

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(b) Electrical or steric barrier

Significant only in O/W emulsions.

In case of non-ionic emulsifying agents, charge may arise due to

(i) adsorption of ions from the aqueous phase or(ii) contact charging (phase with higher dielectric constant is charged

positively)

No correlation between droplet charge and emulsion stability in W/Oemulsions

Steric barrierdehydration and change in hydrocarbon chainconformation.

Emulsions

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(c) Viscosity of the continuous phase

Higher viscosity reduces the diffusion coefficient

Stoke-Einsteins Equation

This results in reduced frequency of collision and therefore

lower coalescence. Viscosity may be increased by adding

natural or synthetic thickening agents.

Further, as the no. of droplets

(many emulsion are more stable in concentrated form than when

diluted.)

Emulsions

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(d) Size distribution of droplets

Emulsion with a fairly uniform size distribution is more stable than

with the same average droplet size but having a wider sizedistribution

(e) Phase volume ratio

As volume of dispersed phase stability of emulsion

(eventually phase inversion can occur)

(f) TemperatureTemperature , usually emulsion stability

Temp affectsInterfacial tension, D,solubility of surfactant,

Brownian motion, viscosity of liquid, phases of interfacial film.

Emulsions

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Preparation of emulsion

Dry gum method

Wet gum method

Bottle method

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Selection of Emulsifiers

Correlation between chemical structure of surfactants and

their emulsifying power is complicated because(i) Both phases oil and water are of variable compositions.

(ii) Surfactant conc. determines emulsifier power as well as thetype of emulsion.

Basic requirements:

1. Good surface activity

2. Ability to form a condensed interfacial film

3. Appropriate diffusion rate (to interface)

G G i i

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1. Type of emulsion determined by the phase in which emulsifier

is placed.

2. Emulsifying agents that are preferentially oil soluble form W/O

emulsions and vice versa.

3. More polar the oil phase, the more hydrophilic the emulsifier

should be. More non-polar the oil phase more lipophilic the

emulsifier should be.

General Guidelines:

G l G id li

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1. HLB methodHLB indicative of emulsification behavior.

HLB 3-6 for W/O

8-18 for O/W

HLB no. of a surfactant depend on which phase of the final emulsion

it will become.

Limitationdoes not take into account the effect of temperature.

General Guidelines

G l G id li

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2. PIT methodAt phase inversion temperature, the hydrophilic

and lipophilic tendencies are balanced.

Phase inversion temperature of an emulsion is determined

using equal amounts of oil and aqueous phase + 3-5%

surfactant.

For O/W emulsion, emulsifier should yield PIT of 20-600C

higher than the storage temperature.

For W/O emulsion, PIT of 10-400

C lower than the storagetemperature is desired.

General Guidelines

G l G id li

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3. Cohesive energy ratio (CER) methodInvolves matching HLBs of oil and emulsifying agents;

also molecular volumes, shapes and chemical nature.

Limitationnecessary information is available only fora limited no. of compounds.

General Guidelines