Colloid chemistry chapter 10

Emulsion – Suspension of liquid droplets dispersed phase of certain size within a second immiscible liquid continuous phase... Classification of emulsions- Based on dispersed phase Oil

Dr Ngo Thanh An Email: ngothanhan@gmail.com

COLLOID CHEMISTRY

Chapter 10 – Emulsion

Emulsion – Suspension of liquid droplets (dispersed

phase) of certain size within a second immiscible liquid (continuous phase)

1 Introduction

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

Beside these two mayor groups of emulsions also more complex emulsion systems are possible (W/O/W and O/W/O).

- Based on size of liquid droplets

0.2 – 50 mm Macroemulsions (Kinetically Stable)

0.01 – 0.2 mm Microemulsions (Thermodynamically Stable)

1 Introduction

Metal cutting oils Margarine Ice cream

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

1 Introduction

Reduction of the surface tension is the key to avoid the coagulation of the emulsion

Substances which reduce the surface tension are called surfactants

In O/W and W/O, surfactants play a role of emulsifying agent

Surfactants must exhibit the following characteristics to be effective as emulsifiers

- Good surface activity

- Should be able to form a condensed interfacial film

- Diffusion rates to interface comparable to emulsion forming time

2 Emulsion agent

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

Nonionic – Polyglycol, Fatty acid esters, Lecithin

Cationic – Quaternary ammonium salts, Amine hydrochlorides

Particle stabilizer

2 Emulsion agent

Particle stabilizer

2 Emulsion agent

Hydrophil area

(= lipophob) Lipophil area (= hydrophob)

Emulsifier is solved in the outer phase

How does a emulsifier look like?

• Reduction of the surface energy

• Generation of steric and electrostatic inhibitions (inhibition of coalescence of the drops)

2 Emulsion agent

Hydrophil head Lipophil chain

water

Working principle of emulsifier

2 Emulsion agent

Micell and reverse micell

2 Emulsion agent

• 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 core l: hydrocarbon chain length

a0: effective head group area

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

2 Emulsion agent

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 The phase in which an emulsifier is more soluble constitutes the continuous phase

Water Oil

Surfactant

Water Oil

Surfactant more soluble in

Wate r

Oil

Surfactant

Water Oil

Surfactant more soluble in

2 Emulsion agent

Surfactant properties – HLB

2 Emulsion agent

Surfactant properties – HLB

2 Emulsion agent

Surfactant properties – HLB

2 Emulsion agent

Surfactant properties – HLB

2 Emulsion agent

Mid Point of Packing

Parameter

P = 1 analogous to HLB 10

At P = 1/ HLB =

10, surfactant has equal affinity for oil and water

Packing Parameter is inversely related to HLB

2 Emulsion agent

Surfactant properties – Krafft temp.

2 Emulsion agent

Surfactant properties – Krafft temp.

2 Emulsion agent

3 STABILITY OF EMULSION

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

3 STABILITY OF EMULSION

(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

(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.)

3 STABILITY OF EMULSION

Emulsions are Kinetically Stable

(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

3 STABILITY OF EMULSION

Emulsions are Kinetically Stable

3 STABILITY OF EMULSION

Types of Physical instability are:

• Creaming, in the laboratory sense, is the migration

more dense they may be than the continuous

phase, and also how viscous or how thixotropic the

• For as long as the particles remain separated, the process is called creaming

3 STABILITY OF EMULSION

Creaming

• Creaming is usually seen as undesirable because it causes difficulties in storage and handling, but it can be useful in special cases, especially where it is desirable

to concentrate an emulsion

• A particular example is in the separation of dairy cream, either to achieve a desired concentration of

butterfat, or to make butter

• Depending on whether the dispersed particles are less dense or more dense than the continuous phase, they may move either to the top of a sample, or to the bottom

3 STABILITY OF EMULSION

Creaming

• the process of migration is called creaming while the particles of the substance remain separated

particles clump) or emulsion breaking (where

particles coalesce)

• One important difference between creaming and the other two processes; unlike flocculation and

breaking, creaming of an emulsion is largely a

simple process to reverse

3 STABILITY OF EMULSION

Creaming

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

kT gH

a   

3 4

3 STABILITY OF EMULSION

Creaming

3 STABILITY OF EMULSION

Emulsion inversion

Based on the Bancroft’s rule, it is possible to change an

emulsion from O/W type to W/O type by inducing changes

Acetic Acid

Water Benzene

Benzene & water - partly miscible,

acetic acid & water - partly miscible

Acetic 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)

3 STABILITY OF EMULSION

Emulsion inversion

Increase T: At a specific temperature, surfactant becomes Oil Soluble across all proportions,

Acetic Acid does not!

Acetic Acid

Water Benzene

Acetic Acid

Water Benzene

Surfactant

Water Oil

Surfactant

Water

Oil Increase in

T, P Increase in T, Electrolyte

3 STABILITY OF EMULSION

Why does Phase Inversion Take Place for system with Surfactants?

Surfactant

Water Oil

Surfactant

Water Oil

Temperature for Non Ionics, Salting out electrolytes for ionics

Bancroft’s Rule: Manifested in Response of

Surfactant Solubility

Temperature for Non Ionics, Salting out electrolytes for ionics

Temperature and electrolytes disrupt the water molecules

around non-ionic and ionic surfactants respectively, altering

surfactant solubility in the process

– Also reflected by change in curvature of the interface

3 STABILITY OF EMULSION

3 STABILITY OF EMULSION

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/O emulsion and vice versa

3 Phase volume ratio

Oil/Water ratio W/O emulsion and vice versa

3 STABILITY OF EMULSION

Emulsion 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

3 STABILITY OF EMULSION

Emulsion inversion

3 STABILITY OF EMULSION

Phase inversion temp PIT

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

4 METHODS OF DESTABILIZING 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

4 METHODS OF DESTABILIZING EMULSIONS

5 MANUFACTURING OF EMULSION

Filter paper

Emulsion Type and Means of

Detection

6 TEST OF EMULSION TYPE

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

properties of the continuous phase

• In this test the emulsion is diluted either with oil or water If the emulsion is

o/w type and it is diluted with water, it will remain stable as water is the dispersion medium" but if it is diluted with oil, the emulsion will break as oil and water are not miscible with each other Oil in water emulsion can easily

be diluted with an aqueous solvent whereas water in oil emulsion can be diluted with a oily liquid.

6 TEST OF EMULSION TYPE

Dilution test

• The basic principle of this test is that

water is a good conductor of

electricity Therefore in case of o/w

emulsion, this test will be positive as

water is the external phase

‘In this test, an assembly is used in which a pair of

electrodes connected to an

electric bulb is dipped into an

emulsion If the emulsion is

o/w type, the electric bulb glows.’

6 TEST OF EMULSION TYPE

Conductivity test

• In this test an emulsion is mixed with a water soluble dye (amaranth) and

observed under the microscope If the continuous phase appears red, it means that the emulsion is o/w type as water is in the external phase and the dye will dissolve in it to give color If the scattered globules appear red and continuous phase colorless, then it is w/o type Similarly if an oil soluble dye (Scarlet red C

or Sudan III) is added to an emulsion and the continuous phase appears red, then it is w/o emulsion.

6 TEST OF EMULSION TYPE

Dye - solubility test

Fluorescence Test:

• If an emulsion on exposure to

ultra-violet radiations shows continuous

fluorescence under microscope,

then it is w/o type and if it shows

only spotty fluorescence, then it is

o/w type.

Cobalt Chloride Test:

• When a filter paper soaked in cobalt

chloride solution is dipped in to an

emulsion and dried, it turns from

blue to pink, indicating that the

emulsion is o/w type.

6 TEST OF EMULSION TYPE

Ref index and filter paper test