Objective

Our objective is to study the effectiveness of different common oils (castor oil, cotton seed oil, coconut oil, kerosene oil, mustard oil) in forming emulsions.

The Theory

What are Emulsions?

Emulsions are an important class of the colloidal system. It is a liquid-liquid colloidal system in which the dispersed phase and dispersion medium are liquids. Emulsions are a mixture of two or more immiscible liquids that are normally immiscible. They have the property to scatter the light passed through them, called Tyndall effect.

If the emulsion scatters all light equally, it becomes white. If the emulsion is dilute, the low wavelength light will be scattered more and it becomes bluer and if it is more concentrated, the longer wavelength light will be more scattered and emulsion becomes more yellow.

A few examples are milk and mayonnaise.

How can we classify Emulsions?

Emulsions can be classified into different types.

1. Oil-in-Water Emulsions: In this emulsion oil is dispersed in water, i.e.. oil is the dispersed phase and water is the dispersion medium. A few examples are milk, foundation cream and vanishing cream.
2. Water-in-Oil Emulsions: In this emulsion water is dispersed in oil, i.e., water is the dispersed phase and oil is the dispersion medium. A few examples are butter and cod-liver oil.
3. Multiple emulsions: In this emulsion, oil-in-water or water-in-oil emulsion is dispersed in another liquid medium. They are of two types:
   1. Oil-in-Water-in-Oil emulsions: In this emulsion, very small globules of oil is dispersed in water globules of water-in-oil emulsion.
   2. Water-in-Oil-in-Water emulsions: In this emulsion, droplets of water is dispersed in the oily phase of oil-in-water emulsion.

Instability in Emulsions

Stability of an emulsion refers its ability to resist the change in its properties. Emulsions are often recognised by their cloudy white appearance, since the substances do not mix together in a unified manner. After a short time, the mixed substances separate into different layers by various manners. This is called the instability of emulsion. There are four different types of instability in emulsions:

• Flocculation: It occurs where there is an attractive force between droplets. They stick together to form clumps or flocs like bunches of grapes. So that the dispersed phase is no longer evenly distributed through the dispersion medium.
• Coalescence: It occurs when small droplets bump into each other and combine together to form a large droplet. As a result, the average droplet size increases and the emulsion become unstable.
• Creaming: It occurs when droplets rise to the top of the emulsion by the influence of buoyancy (an upward force exerted by a fluid that opposes the weight of an immersed object) or by the influence of centripetal force.
• Breaking: When coalescence and creaming combine, the dispersed particles completely separated from the dispersion medium and floats to the top in a single continuous layer.

What is emulsification?

The process of making emulsions is known as emulsification. Emulsions can be obtained by vigorously agitating a mixture of both the liquids. But these emulsions are thermodynamically unstable because the dispersed droplets at once come together and form separate layers. The emulsions therefore need to be stabilised.

How do we stabilise emulsions?

There are several ways by which emulsions can be stabilised. One method is the addition of small quantity of third substances known as emulsifying agents.

What are emulsifying agents?

An emulsifying agent also called emulsifier is a substance that stabilises the emulsion by lowering the interfacial tension between the two immiscible liquids. It is also called stabiliser because it stabilises the emulsion. Soaps and detergents are most commonly used emulsifiers. They coat the drops of the emulsion and check them from coming together and stabilises the emulsion.

For example, soap molecules have a polar head and a non-polar hydrocarbon tail. The polar head is hydrophilic (water loving) in nature and the non-polar tail is hydrophobic (oil loving) in nature. When soap solution is added into an emulsion of oil-in-water, the polar head dissolves in the water phase and the non-polar tail dissolves in the oil droplets there by stabilising the emulsion.

Learning Outcomes

• Students understand the terms colloids, emulsions, Tyndall effect and emulsifying agents.
• Students acquire the skill to perform the experiment using different oils.
• Students understand the effect of stabilising agent in the emulsion of different oils.