An emulsion is a colloidal system in which both, dispersed phase and dispersion medium, are in liquid state. Milk is familiar example of emulsion which is considered as fat emulsified with water. In most cases one of the liquid is oil and other is water. Hence two types of emulsion are normally studied. (1) Oil in water (o/w) emulsion and (2) Water in oil (w/o) emulsion,
Most of the emulsions are unstable. In order to get stable emulsion it is necessary to add another substance known as emulsifier or emulsifying agent. Long chain organic compounds with polar groups such as soaps of various kinds, long chain sulphonic acids, alkyl sulphates work as emulsifiers. They got adsorbed at the interface between the dispersed droplets and dispersion medium in the form of monomolecular layer. It lowers the interfacial tension between oil and water so as to facilitate the mixing of two liquids.
The soap molecules get Hydrocarbons concentrated at the chain interface between water and oil in such a way that the polar ends ( COONa) dip in water and the hydrocarbon chain dip in oil. It results in bringing two liquids in intimate contact and stable emulsion is obtained. Some lyophilic colloids such as proteins, gums, gelatin, agar-agar also act as emulsifiers.
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