Emulsion
Emulsions are liquid-liquid colloidal systems. In other words, an emulsion may be defined as a dispersion of finely divided liquid droplets in another liquid. In simple words, emulsions are a mixture of two or more liquids that are normally immiscible. Generally one of the two liquids is water and the other is immiscible with water like oil. Either liquid can constitute the dispersed phase. Oil and water are the classic example of emulsion.
Types of Emulsions
There are two types of emulsions. One is Oil-in-Water type (O/W type) and other is Water-in-Oil type (W/O type).
Oil-in-Water (O/W) Emulsion: Oil droplets are dispersed throughout a continuous water phase that means oil is dispersed phase and water is dispersed medium or continuous phase. Oil-in-Water type emulsions are lighter, less greasy feel and easily washable with water.
Water-in-oil (W/O) Emulsions: Water droplets are dispersed throughout a continuous oil phase that means water is dispersed phase and oil is dispersed medium or continuous phase. Oil-in-Water type emulsions are heavier, more greasier and provide a more protective barrier for loss of moisture.
Examples of Emulsions
Milk is an emulsion of Oil-in-Water type. Tiny droplets of liquid fat are dispersed in water. Lotions and day creams are the other examples of Oil-in-Water type emulsion.
Stiff greases are emulsions of Water-in-Oil type, water being dispersed in lubricating oil. Other examples are Butter and margarine.
Preparation of Emulsions
The dispersal of a liquid in the form of an emulsion is called emulsification. This can be done by agitating a small proportion of one liquid with the bulk of the other. It is better accomplished by passing a mixture of the two liquid through a colloid mill known as homogenizer.
The emulsions obtained simply by shaking the two liquids are unstable. The droplets of the dispersed phase coalesce and form a separate layer. To have a stable emulsion, small amount of a third substance called the Emulsifier or Emulsifying agent is added during the preparation. This is usually a soap, synthetic detergent, or a hydrophilic colloid.
What is the role of emulsifier?
Emulsions are thermodynamically unstable but can be make kinetically stable through the use of emulsifiers. Egg yolks, mustard, Soy lecithin, and honey are examples of emulsifiers. The emulsifier concentrates at the interface and reduces interfacial tension (surface tension) on the side of one liquid which rolls into droplets.
Soap, for example, is made of a long hydrocarbon tail (oil soluble) with a polar head —COO– Na+ (water soluble). In Oil-in-Water type emulsion, the tail is pegged into the oil droplet, while the head extends into water. Thus, the soap acts as go-between and the emulsified droplets are not allowed to coalesce.
Also read Cleansing Action of Detergent
Properties of Emulsions
Demulsification
Demulsification involves the coalescence of dispersed droplets, leading to phase separation. Emulsions can be broken or demulsified to get the constituent liquids by heating, freezing, centrifuging, or by addition of appreciable amounts of electrolytes. They are also broken by destroying the emulsifying agent.
For example, an oil-water emulsion stabilized by soap is broken by addition of a strong acid. The acid converts soap into insoluble free fatty acids.
Dilution
Emulsions can be diluted with any amount of the dispersion medium. On the other hand the dispersed liquid when mixed with it will at once form a separate layer. This property of emulsions is used to detect the type of a given emulsion.
Applications of Emulsions
Emulsions are found in many daily life products and industrial processes:
Food: Oil-in-water type emulsions are common in food products. Milk, mayonnaise, salad dressings, butter, and ice cream are all emulsions.
Cosmetics: Both type of emulsions are used in cosmetic products depending on the purpose. Lotions and day creams are Oil-in-Water type emulsion while many other personal care products like sunscreens or night creams are Water-in-Oil type emulsion.
Pharmaceuticals: Emulsions are used to create medications, ointments, and creams.
Industry: Paints and coatings are Oil-in-Water type emulsion. Other chemical processes also utilize emulsions.