Dodecane droplets in a continuous phase of water/glycerol mixture.

Sodas: Oil in Water emulsion

Milk: Oil in Water emulsion

Balm: Water in oil emulsion

Mayonnaise: Oil in Water emulsion

EmulsionsEmulsions

Emulsion suitable for intravenous

injection.

Emulsion – Dispersion 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 water

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

Water in Oil in water (W/O/W): Water in Oil emulsion dispersed in water – multiple emulsion

- Based on size of liquid droplets

0.2 – 50 m Macroemulsions

0.01 – 0.2 m Microemulsions

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

Metal cutting oils Margarine Ice cream

Pesticide Asphalt Skin cream

Emulsions encountered in everyday life! Emulsions encountered in everyday life!

Stability of emulsions may be engineered to vary from Stability of emulsions may be engineered to vary from seconds to years depending on application seconds to years depending on application

Stable dispersions of liquids constituting the dispersed phase, in an immiscible liquid constituting the continuous phase is brought about using emulsifying agents such as

Carbohydrates: acacia, tragacanth, agar, chondrus and pectin

Proteins: gelatin, egg yolk and caseinHigh mol wt alcohols: stearyl alcohol, cetyl alcohol, glycery monostearate, cholesterol – w/o stabilisersSurfactants: SPAN, TWEEN, organic soaps ( triethanolamine oleate),Non ionic- pH 3-10, cationic – 3-7, anionic- greater than 8

Emulsifying AgentsEmulsifying Agents

Surfactants

Anionic – Sodium stearate, Potassium laurate

Sodium 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 AgentsCommon Emulsifying Agents

Based on the Bancroft’s rule, many emulsion properties are governed by the properties of the continuous phase

1. Dye test 2. Dilution test3. Electrical conductivity measurements 4. Filter paper test

Tests for Emulsion Type Tests for Emulsion Type

(W/O or O/W emulsions)(W/O or O/W emulsions)

Thermodynamic instability

• ∆ G = Ὺ . ∆ A

• Increase in the surface free energy = interfacial tension X increased surface area

Mechanism of emulsification

• Monomolecular theory

Surfactants

• Reduce interfacial tension

• Forms a protective film around globule

• Ionic surfactant exert repulsion between globules

Mechanism

Multimolecular theory

• Hydrocolloids form multimolecular physical barrier around globules there by prevent coalescence of oil globules

• Acacia, gelatin

Solid particle adsorption theory

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

emulsion.

Reversible process but leads to breaking

Influenced by: Stoke’s equation

V = V = hh = = dd22stst ( (ρρss––ρρoo) g ) g

t 18t 18ηηoo

-globule size

-Viscosity of dispersion medium

-Difference in the densities of dispersed and dispersion medium

Droplets larger than 1 m may settle preferentially to the top or the bottom 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, Δρ - density difference,

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

Creaming can be prevented by homogenization. Also by reducing Δρ, creaming may be prevented.

kTgHa 3

3

4

Creaming of EmulsionsCreaming of Emulsions

Creaming can be reduced/prevented by

• Reducing the globule size by homogenization

• Increasing the viscosity of dispersion medium

• Reducing the difference in densities

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

O/W W/O

1. The order of addition of the phasesW O + emulsifier W/OO W + emulsifier O/W

2. Nature of emulsifierMaking the emulsifier more oil soluble tends to produce a W/O emulsion and vice versa.

3. Phase volume ratioOil/Water ratio W/O emulsion and vice versa

Inversion of Emulsions (Phase inversion)Inversion of Emulsions (Phase inversion)

4. Temperature of the system

Temperature of O/W (polyoxyethylenated nonionic surfactant) 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 sodium cetyl sulfate and cholesterol) to a W/O type upon addition of polyvalent Ca.

Inversion of Emulsions (Phase inversion)Inversion of Emulsions (Phase inversion)

Bancroft's ruleBancroft's rule

Emulsion type depends more on the nature of the emulsifying agent than 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 the continuous phase

In O/W emulsions – emulsifying agents are more soluble in water than in oil (High HLB surfactants).

In W/O emulsions – emulsifying agents are more soluble in oil than in water (Low HLB surfactants).

W/O vs. O/W emulsionsW/O vs. O/W emulsions

Rate of coalescence – measure 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 Emulsions

(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/O emulsions

Steric barrier – dehydration and change in hydrocarbon chain conformation.

Emulsions Emulsions

(c) Viscosity of the continuous phase

Higher viscosity reduces the diffusion coefficient

Stoke-Einstein’s 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 Emulsions

(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 size distribution

(e) Phase volume ratio

As volume of dispersed phase stability of emulsion (eventually phase inversion can occur)

(f) Temperature

Temperature , usually emulsion stability Temp affects – Interfacial tension, D, solubility of surfactant, Brownian motion, viscosity of liquid, phases of interfacial film.

Emulsions Emulsions

Preparation of emulsion

Dry gum method

Wet gum method

Bottle method

Selection of EmulsifiersSelection of Emulsifiers

Correlation between chemical structure of surfactants andtheir emulsifying power is complicated because

(i) Both phases oil and water are of variable compositions.(ii) Surfactant conc. determines emulsifier power as well as the type of emulsion.

Basic requirements:1. Good surface activity2. Ability to form a condensed interfacial film3. Appropriate diffusion rate (to interface)

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:General Guidelines:

1. HLB method – HLB 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.

Limitation – does not take into account the effect of temperature.

General GuidelinesGeneral Guidelines

2. PIT method – At 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 10-400C lower than the storage temperature is desired.

General GuidelinesGeneral Guidelines

3. Cohesive energy ratio (CER) method

Involves matching HLB’s of oil and emulsifying agents; also molecular volumes, shapes and chemical nature.

Limitation – necessary information is available only for a limited no. of compounds.

General GuidelinesGeneral Guidelines