A stable emulsion is one in which the globules retain their initial character like mean size and size distribution and remain uniformly distributed throughout the continuous phase. During storage of emulsions instability is endorsed by creaming, cracking, reversible aggregation and/or irreversible aggregation etc.

1. Cracking or coalescence:

Coalescence is a growth process during which the emulsified particles join to form large particles. When small droplets are merged and large droplets are formed, it suggests that the emulsion will separate completely and cannot be re-
dispersed by shaking. This process is also known as cracking. Any chemical, physical or biological effects that change the nature of the interfacial film of an emulsifying agent may cause cracking. Creaming can be induced by the following methods.

i) Addition of an emulsifying agent of opposite type

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ii) Decomposition or precipitation of emulsifying agent

iii) Addition of a common solvent

iv) Addition of proper preservatives

v) Incorporation of excess of disperse phase

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2. Creaming:

Under the influence of gravity suspended particles or globules tend to upward movement, known as creaming while downward movement of particles or droplets is called sedimentation. Creaming or sedimentation depends on the differences in specific gravity between the phases. If creaming take place without any aggregation, the emulsion can be reconstituted by shaking. Process of creaming is explained by Stoke’s law.

Where,

v = terminal or settling velocity

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d = diameter of globules

Pi = density of disperse phase

p2 = density of continuous phase

η= viscosity of dispersion medium

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g = acceleration due to gravity

Factors that influence the rate of creaming and sedimentation:

i) Globules size: Globules of small size have less tendency to cream.

ii) Viscosity: Higher the viscosity of continuous phase, less creaming

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iii) Density: Less difference in density of two phases means more stability of emulsion.

iv) Temperature: Lower temperature is more suitable for the better stability of emulsion

3. Phase inversion:

It is known that w/o emulsion of benzene in water that was stabilized with sodium stearate will invert to o/w emulsions upon heating and reform w/o emulsions upon cooling. The temperature at which the inversion occurs depends on the emulsifier concentration and is called Phase Inversion Temperature (PIT). An o/w emulsion stabilized by a non-ionic polyoxyethylene-derived surfactant contains oil-swollen miscelles of the surfactant as well as emulsifying oil.

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Containers

Emulsion should be supplied in wide mouthed bottles

Storage

Emulsion should be kept at temperature not exceeding 20°. They should not be allowed to freeze.

Labeling

Comply with general requirements for labeling, in addition the label should indicate following information: quantity of emulsifying agent, preservative, “Shake well before use”, and “Keep in cool place”.

Method of Dispensing:

Triturate the weighed quantity of liquid paraffin and chloroform with the Indian gum, tragacanth the vanillin in mortar and pestle and triturate until a creamy emulsion is formed. Dissolve sodium benzoate in small quantity of purified water and add other additives with continuous stirring.

Use: laxative

Example: To and prepare and dispense castor oil emulsion

Rx

Castor oil 37.5 ml

Acacia in powder 10.0 g

Cinnamon water, sufficient to produce 100.0 ml

Method of Dispensing:

Triturate the weighed quantity of castor oil and the cinnamon water with acacia in mortar and pestle. Add remaining quantity of cinnamon water to produce 100 ml.

Use: Laxative

Example: Prepare and dispense Concentrated Peppermint Emulsion Rx

Peppermint oil 2.0 ml

Polysorbate-20 0.10 ml

Double strength chloroform water 50.0 ml

Purified water, sufficient to make 100.0 ml

Method of Dispensing:

Mix peppermint oil with the polysorbate-20 by shaking and add gradually to double strength chloroform water. Add sufficient purified water to produce 100 ml with shaking.