A system of colloids(is a type of mixture in which one substance is dispersed constantly) in which both the dispersed phase along with the dispersion medium are not mixable( also known as immiscible or partially miscible liquids is called an emulsion. e.g. Milk, butter, oil paints, vanishing cream, cold creams, egg yolk etc. are emulsions.

Most of the time, one of the two liquids is water and the other which is non-mixable with water is designated as oil. The liquid in greater amount is the dispersion medium and the liquid which is in lesser amount or globules is called the dispersed phase.

Emulsifying Agent or Emulsifier:

Emulsions of oil and water are not so table and many times they separate into two layers on standing. To make a stable emulsion a small quantity of another or third substance is added with side two non- mixable liquids. This extra substance is called Emulsifying agent or Emulsifier. Emulsifier creates a protective layer around disperse phase substance and prevent coagulation. e.g. Soaps, Detergents,(i.e. sticking together). Lyophilic (water loving) substances like gelatin, RBC, gum etc.

Types of Emulsions Preparation:

Oil in Water Type of Emulsions (O/W):

In this form of emulsion, oil is in less amount and hence is the dispersed phase and water is in greater amount and hence is the dispersion medium e.g. milk, fats , vanishing creams are oil in water type of emulsion.

Preparation:

The emulsifier is mixed in water and oil is added to it slowly drop by drop with continuous stirring.

Characteristics of Oil in Water Emulsions:

• Oil is the smaller amount and water is the dispersion medium.
• If water is added it is mixed with the emulsion.
• If oil is put into it, it is not miscible with the emulsion.
• Putting a small amount of electrolyte makes emulsion more conducting.
• Water is in a continuous phase. 
• Basic sulphates of metals, are water soluble alkali metal soaps are used as emulsifiers.

Water in Oil Type (W/O):

In this sort of emulsions, water is in small amounts hence is the dispersed phase and m. e.g. Hollandaise sauce Cod liver oil in which particles of water are dispersed in oil, Cold creams.

Preparation:

The emulsifier is mixed in oil and water is put into it drop by drop with continuous stirring.

Characteristics of Water in Oil Emulsions:

• Water is the dispersed phase and oil is in greater amounts and is the dispersion medium.
• If oil is put into it is miscible with the emulsion.
• If water is added it is not mixed with the emulsion.
• Addition of a little amount of electrolyte does not make any changes in the conductive nature of emulsions.
• Oil is the continuous phase here.
• Soaps not soluble in water such as those of Zn, Al, Fe, alkaline earth metals are used as emulsifiers.

Methods of Identification of Type of Emulsions:

Dye Solubility Test:

In this test, an emulsion miscible with a water-soluble dye is taken and mixed and is observed under a microscope. If the regular phase appears red, it indicates that the emulsion is o/w type as water is in the outer phase and the dye will mix in it to give colour. If the scattered droplets appear red and in regular phase colourless, then it is w/o type. Accordingly, if an oil-dissolvable dye (Scarlet red C or Sudan III) is mixed to an emulsion and the continuous phase appears red, then we can say that it is w/o emulsion.

Viscosity Test:

The viscosity of water in oil type emulsion is greater than the viscosity of oil in water type emulsion. So, we can easily point out once we measure the viscosity of the emulsion.

Electrical Conductive Test:

The crux principle of this test is that water is a better conductor of electricity. In the case of o/w emulsion, this test will be positive as water is the regular phase. If an electrolyte like NaCl( common salt) is dissolved to oil in water type emulsion, its conductivity increases exponentially.

Properties of Emulsions:

Precipitation:

In many of the emulsions, the size of the substance in smaller amounts i.e in droplets is larger than the particles found in sols. These droplets can be precipitated by adding suitable electrolytes.

De-emulsification:

Separation of two liquid phases (two distinct layers) from emulsions due to the removal of an emulsifier by any means is called de-emulsification or de-emulsification. It can be done by heating, freezing, centrifuging or by addition of electrolytes.

Characteristics of Emulsions:

• It is the system of colloids in which the dispersed phase and the dispersion medium both are in the liquid phase.
• It is a moving liquid.
• Droplets of one liquid dissolved in another liquid solution.
• They don’t show a tendency to absorb a liquid or to swell.

• The emulsifying agent is always needed.

• It is categorised as oil in water and water in oil.

• They show effects such as Tyndall effect, Brownian movement and electrophoresis.

Conclusion

A solution colloids in which the dispersed phase, and the dispersion medium, are both in the liquid phase. As emulsion is a colloid, whose particle size can range between the true solution and suspension. It is mainly of two types: water in oil and oil in water. They form the major components on most of the things we use today.