Emulsions: Liquid-Liquid Colloids

“The colloidal systems in which fine droplets of one liquid are dispersed in another liquid are called emulsions, the two liquids otherwise being mutually immiscible.” 

If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one liquid in the other is obtained which is called emulsion.

Types of emulsion

Depending upon the nature of the dispersed phase, the emulsions are classified as;

i. Oil-in-water emulsions (O/W): 

The emulsion in which oil is present as the dispersed phase and water as the dispersion medium (continuous phase) is called an oil-in-water emulsion. Milk and vanishing cream are examples of the oil-in-water type of emulsion. In milk liquid fat globules are dispersed in water.

ii. Water-in-oil emulsion (W/O):

The emulsion in which water forms the dispersed phase, and the oil acts as the dispersion medium is called a water-in-oil emulsion. These emulsions are also termed oil emulsions. Butter, cream and cod liver oil, are examples of this type of emulsion.

Properties of emulsion

• Emulsions of oil in water are unstable and sometimes they separate into two layers on standing. For stabilisation of an emulsion, a third component called emulsifying agent is added. The emulsifying agent forms an interfacial film between suspended particles and the medium. The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps, etc., and for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack, etc.
• Emulsions can be diluted with any amount of the dispersion medium. On the other hand, the dispersed liquid when mixed forms a separate layer.
• The droplets in emulsions are often negatively charged and can be precipitated by electrolytes, containing polyvalent metal ions indicating the negative charge on the globules.
• Emulsions show all the characteristic properties of colloidal solution such as Brownian movement, Tyndall effect, electrophoresis etc.
• Emulsions can be broken into constituent liquids by heating, freezing, centrifuging, etc. This process is also known as demulsification.
• The size of the dispersed particles in emulsions is larger than those in the sols. It ranges from 1000 Å to 10,000 Å. However, the size is smaller than the particles in suspensioins.