06 emulsion

8/2/2019 06 Emulsion

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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

Emulsions

Emulsion

suitable for

intravenous

injection.



Outline

• Introduction

• Types of emulsions

• Emulsifying agents

• Tests for emulsion types

• Emulsion Stability

• Phase Inversion, Creaming

• Emulsion Breaking



Introduction

Emulsion – Suspension of liquid droplets (dispersed phase) of

certain size within a second immiscible liquid (continuous

phase).

Classification of emulsions

- Based on dispersed phaseOil in Water (O/W): Oil droplets dispersed in water

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

- Based on size of liquid droplets

0.2 – 50 mm Macroemulsions (Kinetically Stable)

0.01 – 0.2 mm Microemulsions (Thermodynamically Stable)



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



Stable suspensions of liquids constituting the dispersed

phase, in an immiscible liquid constituting the continuousphase is brought about using emulsifying agents such as

surfactants

Surfactants must exhibit the following characteristics to beeffective as emulsifiers

- Good surface activity- Should be able to form a condensed interfacial film

- Diffusion rates to interface comparable to emulsion forming

time

Emulsifying Agents



Surfactants

Anionic – Sodium stearate, Potassium laurateSodium dodecyl sulfate, Sodium sulfosuccinate

Nonionic – Polyglycol, Fatty acid esters, Lecithin

Cationic – Quaternary ammonium salts,

Amine hydrochlorides

Solids

Finely divided solids with amphiphilic properties such assoot, silica and clay, may also act as emulsifying agents

(Pickering Emulsions: Attribute of high stability)

Common Emulsifying Agents



• Conceptual framework that relates molecular parameters

(head group area, chain length and hydrophobic tail

volume) and intensive variables (temperature, ionic

strength etc.) to surfactant microstructures

• Critical Packing Parameter /

Packing Parameter

v: Volume of hydrocarbon corel: hydrocarbon chain length

a0: Effective head group area

Surfactant Packing Parameter

CPP or P v

l a0



v: Volume of hydrocarbon chain= 0.027(nc + nMethyl)

l: hydrocarbon chain length= 0.15 + 0.127nc

Where nc = number of carbon atoms without the methyl

group

nMethyl = number of methyl groups

ao: Effective head group area: difficult to calculate.

Surfactant Packing Parameter

CPP or P v

l a0



Packing Parameter is inversely related to HLB

Mid Point of

Packing ParameterP = 1

analogous to

HLB 10

At P = 1/ HLB = 10,

surfactant has equa

affinity for oil and

water



Bancroft’s Rule:Relation to HLB & CPP of Surfactant

Surfactant

WaterOil

Surfactant

WaterOil

Surfactant more soluble in

water (CPP < 1, HLB > 10)

O/W emulsion

Surfactant more soluble in oil

(CPP > 1, HLB < 10)W/O emulsion



Bancroft’s Rule:Relation to HLB & CPP of Surfactant

Surfactant

WaterOil

Surfactant

WaterOil


water (CPP < 1, HLB > 10)

O/W emulsion


oil (CPP > 1, HLB < 10)

W/O emulsion

Packing Parameter = 1

Microemulsion



Based on the Bancroft’s rule, many emulsion properties are

governed by the properties of the continuous phase

1. Dye test

2. Dilution test

3. Electrical conductivity measurements

4. Refractive index measurement

5. Filter paper test

Tests for Emulsion Type

(W/O or 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(Analogous to stable foam bubbles)

Mixed surfactant system preferred over single surfactant.

(Lauryl alcohol + Sodium lauryl sulfate: hydrophobic interactions)NaCl added to increase stability (electrostatic screening)

Emulsions are Kinetically Stable!



(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 chargedpositively)

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

emulsions

Steric barrier – dehydration and change in hydrocarbon chainconformation.




(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.)




(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) TemperatureTemperature , usually emulsion stability

Temp affects – Interfacial tension, D, solubility of surfactant,

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




Phase Inversion in Emulsions

Bancroft'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.

Based on the Bancroft’s rule, it is possible to change anemulsion from O/W type to W/O type by inducing changes

in surfactant HLB / CPP.

In other words...

Phase Inversion May be Induced.

C id t f 2 i i ibl d 1 i ibl i f li id



Consider systems of 2 immiscible and 1 miscible pairs of liquids

Acetic Acid

WaterBenzene

Surfactant

WaterOil

Acetic acid & water are miscible in

all proportions

Benzene & water - partly miscible,

acetic acid & water - partly miscibleAcetic acid added to a mixture of

benzene & water, preferentially

partitions into water (slope of tie line)

Surfactant and water are miscible

in all proportions

Oil and water - partly miscible,

surfactant and oil - partly miscible

Tie line

Surfactant added to a mixture of oil

& water, preferentially partitions

into water (slope of tie line)

I T At ifi t t f t t b Oil



Increase T: At a specific temperature, surfactant becomes Oil

Soluble across all proportions, Acetic Acid does not!

Acetic Acid

WaterBenzene

Acetic Acid

WaterBenzene

Surfactant

WaterOil

Surfactant

WaterOil

Increase inT, P

Increase in T,

Electrolyte



Why does Phase Inversion Take Place for system with Surfactants?

Surfactant

WaterOil

Surfactant

WaterOil

O/W emulsion W/O emulsion

Temperature for Non Ionics, Salting out electrolytes for ionics

B ft’ R l M if t d i R f S f t t S l bilit



Bancroft’s Rule: Manifested in Response of Surfactant Solubility

O/W emulsion W/O emulsion

Temperature for Non Ionics, Salting out electrolytes for ionics

Temperature and electrolytes disrupt the water moleculesaround non-ionic and ionic surfactants respectively, altering

surfactant solubility in the process

– Also reflected by change in curvature of the interface



O/W W/O

1. The order of addition of the phases

W O + emulsifier W/O

O 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 ratio W/O emulsion and vice versa

Inversion of Emulsions (Phase inversion)



C i f E l i



Droplets larger than 1 mm 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. This is achieved by producing

a polyphase emulsion

kT gH a 3

3

4

Creaming of Emulsions



Methods of Destabilizing Emulsions

1. 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. Heating

Heating 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 intolong streamers)

• For O/W, electrophoretic migration of charged

groups to one of the electrodes. Ex. Removing tracesof lubricating oil emulsified in condensed water.

Methods of Destabilizing Emulsions



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 l G id li



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



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 Guidelines

G l G id li



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 10-400

C lower than the storagetemperature is desired.

General Guidelines

G l G id li



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

also molecular volumes, shapes and chemical nature.

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

General Guidelines

06 emulsion

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