Emulsions are colloidal solutions in which disperse phase as well as dispersion medium is both liquids. Emulsions can be broadly classified into two types:
(i) Oil in water (O/W type) emulsions: in this type of emulsions, oil acts disperse phase and water acts as dispersion medium. Some examples of this type of emulsions are milk, vanishing cream, etc.
(ii) Water in oil (W/O type) emulsions: in this type of emulsions, water acts as disperse phase and oil acts as dispersion medium. Some examples are cold cream, butter, cod liver oil etc.
The two types of emulsions can be interconverted simply by changing the proportion of the dispersed phase and dispersion medium. For example, O/W type of emulsion can be converted W/O type by adding excess of oil to O/W emulsion.
Detection of emulsions
The below mentioned tests may be applied to distinguish between the two types of emulsions:
(i) Dye test: to the emulsion some oil soluble dye is added. If the background becomes coloured, the emulsion is water in oil type and if the droplets become coloured, the emulsion is oil in water type.
(ii) Dilution test: if the emulsion can be diluted with water, this indicates that water act as the dispersion medium and the emulsion is of oil in water type. If the added water forms a separate layer, then in that case the emulsion is water-in-oil type.
Preparation of emulsions
The process of making an emulsion known as emulsification. Emulsions may be obtained by vigorously agitating a mixture of both the liquids. But this gives an unstable emulsion the dispersed drops at once come together and form a separate layer. To stabilize an emulsion, the addition of a small quantity of the third substance known as emulsify agent or emulsifier is essential. The emulsified agents form an interfacial film between suspended particles and the dispersion medium. For example, soaps and detergents are frequently used as emulsifiers. They coat the drops of an emulsion and check them from coming together thereby establishing the emulsion. The principle emulsifying agent for W/O type emulsions are heavy metal salts of fatty acids, long chain alcohols, lamp black, etc. the emulsifying agents used for O/W type emulsions are proteins, gums, natural and synthetic soaps, etc.