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.
Identification of Types of Emulsion
The two types of emulsions can be identified by the following tests.
Dye Test:
A small amount of dye soluble in oil is added to the emulsion. The emulsion is shaken well. The aqueous emulsion will not take the colour whereas oily emulsion will take up the colour of the dye.
Viscosity Test
Viscosity of the emulsion is determined by experiments. Oily emulsions will have higher value than aqueous emulsion.
Conductivity Test
Conductivity of aqueous emulsions are always higher than oily emulsions.
Spreading Test
Oily emulsions spread readily than aqueous emulsion when spread on an oily surface.
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.
Test Your Knowledge
Emulsion MCQs
1. Which of the following best defines an emulsion?
A. A mixture of a solid dispersed in a liquid.
B. A dispersion of finely divided liquid droplets in another liquid.
C. A solution of two miscible liquids.
D. A gas dispersed in a liquid.
View Answer
Option B is correct answer.
A dispersion of finely divided liquid droplets in another liquid.
2. In an Oil-in-Water (O/W) emulsion, which substance acts as the dispersed phase?
A. Water
B. Oil
C. A solid emulsifier
D. A gas
View Answer
Option B is correct answer.
Oil is dispersed phase and water is dispersed medium or continuous phase.
3. Which of the following is an example of a Water-in-Oil (W/O) type emulsion?
A. Milk
B. Lotions
C. Butter
D. Day creams
View Answer
Option C is correct answer.
Butter and margarine" as examples of Water-in-Oil type emulsions.
4. What is the primary role of an emulsifier in an emulsion?
A. To increase the interfacial tension between the two liquids.
B. To make the emulsion thermodynamically unstable.
C. To reduce interfacial tension and prevent droplets from coalescing.
D. To change the chemical composition of the dispersed phase.
View Answer
Option C is correct answer.
The emulsifier concentrates at the interface and reduces interfacial tension (surface tension) on the side of one liquid which rolls into droplets. and the emulsified droplets are not allowed to coalesce.
5. Which of the following methods can be used for demulsification?
A. Adding a strong acid to a soap-stabilized oil-water emulsion.
B. Stirring the emulsion vigorously for a long time.
C. Increasing the concentration of the emulsifying agent.
D. Diluting the emulsion with the dispersed liquid.
View Answer
Option A is correct answer.
Emulsions can be broken or demulsified 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.
6. How can the type of a given emulsion be detected using the dilution property?
A. By checking if it becomes clearer when heated.
B. By observing if it can be diluted with any amount of the dispersion medium without separation.
C. By adding a strong acid and checking if it breaks.
D. By measuring its viscosity at different temperatures.
View Answer
Option B is correct answer.
Emulsions can be diluted with any amount of the dispersion medium. This property of emulsions is used to detect the type of a given emulsion.
7. Which type of emulsion provides a more protective barrier for loss of moisture?
A. Oil-in-Water (O/W) emulsions
B. Water-in-Oil (W/O) emulsions
C. Both types equally
D. Neither type provides a protective barrier
View Answer
Option B is correct answer.
Water-in-Oil type emulsions are heavier, more greasier and provide a more protective barrier for loss of moisture.
8. Which of the following is NOT an application of emulsions?
A. Food products like mayonnaise.
B. Cosmetics like sunscreens.
C. Pharmaceuticals for ointments.
D. Pure metals production.
View Answer
Option D is correct answer.
Food, Cosmetics, Pharmaceuticals, and Industry (Paints and coatings, other chemical processes) are the applications of the emulsion, but not pure metals production.
9. The dispersal of a liquid in the form of an emulsion is called:
A. Demulsification
B. Homogenization
C. Emulsification
D. Coalescence
View Answer
Option C is correct answer.
The dispersal of a liquid in the form of an emulsion is called emulsification.
10. Why are emulsions obtained simply by shaking two immiscible liquids often unstable?
A. The dispersed phase solidifies quickly.
B. The droplets of the dispersed phase coalesce and form a separate layer.
C. The continuous phase evaporates rapidly.
D. They instantly become thermodynamically stable.
View Answer
Option B is correct answer.
The emulsions obtained simply by shaking the two liquids are unstable. The droplets of the dispersed phase coalesce and form a separate layer.