EMULSIONSEMULSIONS

Introduction (11.1-11.4) Emulsifying agents Types of emulsions Tests for emulsion types Emulsion Stability (11.5-11.6) Phase Inversion, Creaming Emulsion Breaking

IntroductionIntroductionEmulsion Suspension of liquid droplets of certiain size

within a second immiscible liquid.

Stability of an emulsion may vary from a few seconds to several years.

Classification of emulsions based on size of liquid droplets0.2 50 m Macroemulsions0.01 0.2 m Microemulsions

ApplicationsApplicationsAgricultural emulsions pesticides

Bituminous emulsions surface coatings, roads, highways

Cosmetics lotions, foundation creams, cold creams, shaving creams

Paints and Polishes

Metal cutting oils

Food margarine, ice cream.

Emulsifying AgentsEmulsifying Agents

Surfactants which exhibit following characteristics

Good surface activity

Should be able to form a condensed interfacial film

Diffusion rate to interface comparable to emulsion forming time

Examples of Emulsifying AgentsExamples of Emulsifying AgentsAnionic Sodium stearate

Potassium laurateSodium dodecyl sulfateSodium sulfosuccinate

Nonionic PolyglycolFatty acid estersLecithin

Cationic Quaternary ammo. SaltsAmine hydrochlorides

Finely divided solids may also act as emulsifying agents e.g. soot and clay (continuous phase wets the powder)

Types of EmulsionsTypes of Emulsions Based on the nature of the dispersed phase, emulsions

can be of two types Oil in Water (O/W) or Water in Oil (W/O)

O/W dispersion of a water immiscible liquid (always called the oil) in an aqueous phase. Oil is the discontinuous or inner phase and water is the continuous or outer phase

W/O dispersion of water or an aqueous solution in a water immiscible liquid

Bancroft's ruleBancroft's ruleEmulsion type depends more on the emulsifying agent than on the relative proportions of oil or water present or the methodology of preparing emulsion. In general O/W emulsions emulsifying agents that are more soluble in water than in oil.

W/O emulsifying agents that are more soluble in oil than in water phase.

Tests for Emulsion Type (W/O or O/W)Tests for Emulsion Type (W/O or O/W)

Based on the fact many emulsion properties are similar to continuous phase properties

1. Dye test2. Dilution test3. Electrical conductivity measurements4. Refractive index measurement5. Filter paper test

Emulsion StabilityEmulsion StabilityRate of coalescence measure of emulsion stability. It

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

mechanical stability, surfactant film with strong lateral intermolecular forces and high film elasticity is desired.(consider similarity between emulsion droplets and bubbles in a foam). Mixture of two or more surfactants preferred over a simple surfactant. Lauryl alcohol + Sodium lauryl sulfate.NaCl added to increase stability.

(b) Electrical or steric barrierSignificant 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.

(c) Viscosity of the continuous phaseHigher viscosity reduces the diffusion coefficient

Which results in reduced frequency of collision and therefore lesser coalescence.

Viscosity can be increased by adding natural or synthetic thickening agents.

Also as the no. of droplets(many emulsion are more stable in concentrated form than

when diluted.)

(d) Size distribution of dropletsEmulsion 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 ratioAs volume of dispersed phase stability of emulsion (eventually phase inversion can occur)

(f) TemperatureTemperature , usually emulsion stability Temp affects Interfacial tension, solubility of surfactant, thermal agitation, viscosity of liquid, phases of interfacial film.

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

4. Phase in which the emulsifying agent is dissolvedDissolving hydrophilic surfactants in the aqueous phase appears to favor O/W emulsion formation.

5. Temperature of the systemTemperature of O/W (polyoxyethylenated nonionic surfactant) makes the emulsifier more hydrophobic and the emulsion may invert to W/O.

6. Addition of electrolytes and other additives.Strong electrolytes to O/W (stabilized by ionic surfactants) may invert to W/OExample. Inversion of O/W emulsion (stabilized by sodium cetyl sulfate and cholesterol) to a W/O type upon addition of polyvalent Ca.

Creaming of EmulsionsCreaming of EmulsionsDroplets 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 emulsionProbability of creaming can be reduced if

a - droplet radiusWP density differenceH height of the vessel in cmCreaming can be prevented by homogenization. Also by

reducing WP, creaming may be prevented. This is achieved by producing a polyphase emulsion

Add water to a W/O emulsion inverts to a O/W

Methods of Emulsion BreakingMethods of Emulsion Breaking1. Physical methods

(i) Centrifuging(ii) Filtration media pores preferentially wetted by the

continuous phase(iii) Gently shaking or stirring(iv) Low intensity ultrasonic vibrations

2. HeatingHeating to ~ 700C will rapidly break most emulsions.

3. Electrical methods Most widely used on large scale

20 kV results in coalescence of entrained water droplets (W/O) e.g. in oil field emulsions and jet fuels. (mechanism deformation of water drops into long streamers)

For O/W, electrophoretic migration of charged groups to one of the electrodes. Ex. Removing traces of lubricating oil emulsified in condensed water.

Selection of EmulsifiersSelection of EmulsifiersCorrelation 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)

General Guidelines:General Guidelines:

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.

Selection methodsSelection methods (11.8(11.8--11.9)11.9)

1. HLB method HLB indicative of emulsification behavior.

HLB 3-6 for W/O8-18 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.

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.

3. Cohesive energy ratio (CER) methodInvolves matching HLBs of oil and emulsifying agents; also molecular volumes, shapes and chemical nature.

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