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“An Emulsion is a colloidal solution in which both dispersion medium as well as dispersed phase are liquids. (The liquids should be Immiscible)”.
Emulsions prepared by mere shaking of two liquids or by passing the mixture through a colloidal mill, called the Homogenizer, are unstable and tend to separate into two layers on standing. Thus, in order to get a stable emulsion, small quantities of certain other substances are added during preparation. The substances thus added to stabilize the emulsions are called Emulsifiers or Emulsifying Agents. The commonly used emulsifying agents are soaps, proteins, long chain sulphonic acids, basic sulphates of Fe, Cu, Ni etc.

Role of Emulsifier

                  The role of emulsifier can be explained by taking the example of soap as an Emulsifier. Soaps are sodium or potassium salts of higher fatty acids. A molecule of soap consists of two parts, the long hydrocarbon part (R–) which is oil soluble and the polar group COO- Na+ which is water soluble. Thus when oil and water mixture in presence of soap is shaken vigrously, the oil drop is surrounded by soap solution, the R – part of soap remains in oil and COO- Na+ part remains in water. Thus soap molecules are concentrated over the surface of oil drop. As a result interfacial tension between water and oil decreases and hence they do not coagulate or separate.

Types of Emulsion
Emulsion is colloidal solution of two immiscible liquids. Generally, one of the two liquids is water and the other which is immiscible with water, is designated as oil. Either of two liquids can constitute the dispersed phase. Thus there are two types of Emulsions.
i).      Oil-in-water type (o/w type)
ii).     Water-in-oil type (w/o type)
         i).      Oil-In-Water Emulsions
In this type of emulsions, oil acts as disperse phase and water acts as dispersion medium. Some examples of this type of emulsion are milk, vanishing cream etc.
         ii).     Water-In-Oil Emulsions
In this type of emulsions, water acts as dispersed phase and oil acts as dispersion medium. For example, butter, cod liver oil, cold cream etc.

Identification of Emulsions
The following tests may be employed to distinguish between 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 becomes coloured the emulsion is oil-in-water type.
(ii).    Dilution Test
If the emulsion can be diluted with water, this indicates that water is dispersion medium and the emulsion is of oil-in-water type. In case the water added forms a separate layer than emulsion is water-in-oil type.

Applications of Emulsions
Some important applications of emulsions are as follows:-
(1).    Cleansing Action of Soap
Washing action of soap is due to emulsification of grease, dirt and taking it away in water.
(2).    Digestion of Fats
Digestion of fats in the intestines is aided by emulsification. A little of fat forms a sodium soap with alkaline solution in the intestines and this soap emulsifies the rest of fat. It is easier for digestive enzymes to carry out their functions on emulsified fat.
(3).    In Medicines
A wide variety of pharmaceutical preparations are emulsions. For example: cod liver oil, disinfectant phenyl results in emulsion when poured in water.
(4).    In Metallurgical Operations
Emulsions play an important role in industry. The metal ores are concentrated with froth-floatation process which involves the treatment of the pulversied ore in emulsion of pine oil.

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