# Evaluation of Emulsions

## Evaluation of Emulsions

Learning Objectives

At the end of this lecture, student will be able to

• Identify instabilities in emulsions and suggest suitable remedial measures

• Explain the various methods used for evaluation of emulsions

## Instabilities in Emulsions

Instabilities seen in emulsion can be grouped as:

1. Cracking

2. Creaming

3. Phase inversion

1. Creaming

• Separation of an emulsion into 2 regions

• One region is richer in the disperse phase than the other

• Not a serious instability

• Uniform dispersion on shaking

• But creaming can lead to cracking

Types of Creaming

1. Upward creaming

2. Downward creaming

1. Upward creaming

– If the disperse phase is less dense than the continuous phase

– The velocity of sedimentation becomes negative

– Seen in O/W emulsions

2. Downward creaming

– If the internal or disperse (aqueous) phase is heavier than the external or continuous (oil) phase

– The globules settle

– Seen in W/O emulsions

Factors governing creaming

Stoke’s law is used to express the velocity of sedimentation

where,

V = velocity in cm/sec {rate of creaming}

r = radius of the globules

d = diameter of the globules in cm

Ps = density of dispersed phase

Po = density of dispersion medium

g = gravitational constant

n= viscosity of the dispersion medium

Causes of Creaming

• Increase in the diameter of the globules.

• Decrease in the viscosity of the external or continuous phase.

• Increase in density differences between the disperse phase and dispersion medium.

• Variations in storage temperature

Can the emulsion be saved?

• The emulsion will reform on shaking.

• Reversible process

• But creaming can lead to cracking

Prevention of Creaming

• Decreasing the diameter of the oil globules — homogenization

• Increasing the viscosity of the external / continuous phase — addition of a thickening agent.

• Reduction in density differences between the 2 phases— using phases which have similar density

•Storage in a cool place

2. Cracking

• Globules of the disperse phase coalesce and separate into a different layer

• Redispersion cannot be achieved by shaking

• The preparation is no longer an emulsion.

Causes of Cracking

1. Addition of emulsifying agents of the opposite type:

Soaps of monovalent metals – O/W emulsions and soaps of divalent metals – W/O emulsions.

2. Gums, proteins – gelatin & casein are insoluble in alcohol – if alcohol is added, the emulgent is precipitated – cracking occurs.

3. Decomposition of the emulsifying agent due to microbial action

5. Oil is Rancid

6. Change of storage temperature

Can the emulsion be saved?

• The emulsion will not reform on shaking

• It is an irreversible process.

Prevention of Cracking

• Choose the correct emulsifying agent

• Avoid use of common solvent

• Use of preservatives

• Maintain constant temperature conditions for storage

• Use fresh oil for preparation

3. Phase Inversion

• Oil-in-water emulsion changes to a water-in-oil emulsion

or

• Water-in-oil emulsion changes to oil-in-water emulsion.

Causes of Phase Inversion

• Addition of the wrong emulsifying agent

E.g. If calcium chloride is added to an O/W emulsion stabilized by sodium soap, phase reversal may occur resulting in W/O emulsion.

• Increase in concentration of disperse phase beyond 60%

Can the emulsion be saved?

• The emulsion will not reform on shaking

• It is an irreversible process

Prevention of Phase Inversion

• Using the proper emulsifying agent in adequate concentrations

• Maintaining the concentration of dispersed phase between 30 to 60 %

• Storing the emulsion in a cool place.

Evaluation of Emulsions

1. Macroscopic examination: Calculating the volume of the creamed or separated part of the emulsion to the total volume.

2. Globule size analysis- By microscopic examination / electronic particle counting devices such as coulter counter / by Laser diffraction.

3. Viscosity changes: Brookefield’s Viscometer.

## Preservation of Emulsions

1) Preservation from microorganisms: Use suitable preservatives

2) Preservation from oxidation: Use anti- oxidants

## Labeling for Emulsions

SHAKE WELL BEFORE USE

Summary

1) Instabilities in emulsions

– Creaming: Separation of an emulsion into 2 regions and one region is richer in the disperse phase than the other

– Cracking: Separation on an emulsion into two separate layers

– Phase Inversion: Conversion of O/W emulsion into W/O emulsion and vice versa

2) Preservation of emulsions- Using preservatives

3) Labeling of emulsions- Shake Well Before Use

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