The liquid-liquid colloidal dispersions are called emulsions. An emulsion may be defined as follows:
The colloidal dispersion of two immiscible liquids in which one liquid acts as the dispersion medium and the other as dispersed phase is called an emulsion.
Types of emulsion: Depending upon the nature of dispersed phase, emulsions can be classified into following two types.
(i) Oil-in-water (O/W) type emulsions: In oil-in-water emulsions, an oil acts as the dispersed phase while water acts as the dispersion medium. The most common example of oil in water type emulsion is milk which consists of liquid fat globules dispersed in water.
(ii) Water-in-oil (W/O) type emulsions: In water-in-oil type emulsions, water acts as the dispersed phase, whereas oil acts as the dispersion medium. This type of emulsions is also referred to as oil emulsions. Cod liver oil emulsion is a typical example of this type of emulsions in which water is dispersed in cod liver oil. The two types of emulsions are diagrammatically shown in figure below.
(a) Oil-in-water type emulsion (b) Water-in-oil type emulsion
Preparation of emulsions: Emulsions are usually prepared by vigorously mixing the two liquids by using either a high speed mixing machine or by using ultrasonic vibrators. The process is known as emulsification. Since the two liquids used for the preparation of an emulsion are completely immiscible, a stabilizing substance, known as emulsifying agent or emulsifier is required to stabilize the resulting emulsion. The emulsifier is added along with the component liquids. In the absence of emulsifying agent, the dispersed phase particles of colloidal size combine together resulting in the breaking up of emulsion into two separate layers. Some of the important emulsifying agents are soaps, detergents, proteins, gums and agar. Among these, soaps and detergents are most commonly used emulsifiers.
Role of emulsifier: The emulsifiers for a protective film around the oil droplets dispersed in water. This prevents them to come closer and to coalesce, i.e. to combine together. Thus, the emulsion gets stabilized. For example, let us consider the role of soap which acts as an emulsifier for an oil-in-water emulsion. When soap is added to an o/w emulsion, the soap molecule (RCOO-Na+) arrange themselves in such a way that the polar end groups dip in water whereas the hydrocarbon chains dip in oil droplet as shown in the figure below. Thus soap molecules get concentrated over the surface of the oil droplet and form a protective film. This decreases the interfacial between oil and water and the emulsion gets stabilized.
Emulsification of an oil droplet by soap
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