A colloidal system in which both the dispersed phase as well as dispersion medium are immiscible or partially miscible liquids is called an emulsion.
e.g. Milk, cod liver oil, oil paints, vanishing cream, cold creams, etc.
Generally, one of the two liquids is water and the other which is immiscible with water is designated as oil.
The liquid in excess is the dispersion medium and the liquid which forms droplets or globules is the dispersed phase.
Emulsifying Agent or Emulsifier:
Emulsions of oil and water are unstable and sometimes they separate into two layers on standing. To obtain a stable emulsion a small quantity of third substance is added along with two immiscible liquids. This third substance is called Emulsifying agent or Emulsifier.
Emulsifier forms a protective layer around disperse phase droplets and prevent coagulation.
e.g. Soaps, Detergents, Lyophilic substances like gelatin, gum etc.
Types of Emulsions and Their Preparation:
Oil in Water Type (O/W):
In this type of emulsion, oil is the dispersed phase and water is the dispersion medium.
e.g. Milk is (fats) oil in water type of emulsion, vanishing creams.
Preparation: The emulsifier is dissolved in water and oil is added to it drop by drop with continuous agitation.
Characteristics of Oil in Water Emulsions:
Oil is the dispersed phase and water is a dispersion medium.
If water is added it is miscible with the emulsion.
If oil is added it is not miscible with the emulsion.
Addition of small amount of electrolyte makes emulsion conducting.
Water is a continuous phase.
Basic metal sulphates, water-soluble alkali metal soaps are used as emulsifiers.
Water in Oil Type (W/O):
In this type of emulsions, water is the dispersed phase and oil is the dispersion medium.
e.g. Cod liver oil in which particles of water are dispersed in oil, Cold creams.
Preparation: The emulsifier is dissolved in oil and water is added to it drop by drop with continuous agitation.
Characteristics of Water in Oil Emulsions:
Water is the dispersed phase and oil is the dispersion medium.
If oil is added it is miscible with the emulsion.
If water is added it is not miscible with the emulsion.
Addition of small amount of electrolyte does not make emulsion conducting.
Oil is a continuous phase.
Water-insoluble soaps 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 is mixed with a water-soluble dye (amaranth) and observed under the microscope. If the continuous phase appears red, it means that the emulsion is o/w type as water is in the external phase and the dye will dissolve in it to give colour. If the scattered globules appear red and continuous phase colourless, then it is w/o type.
Similarly, if an oil-soluble dye (Scarlet red C or Sudan III) is added to an emulsion and the continuous phase appears red, then it is w/o emulsion.
The viscosity of water in oil type emulsion is more than the viscosity of oil in water type emulsion.
Electrical Conductive Test:
The basic principle of this test is that water is a good conductor of electricity. In the case of o/w emulsion, this test will be positive as water is the continuous phase. If electrolyte like NaCl is added to oil in water type emulsion, its conductivity increases greatly.
Water in oil type emulsion spread on the surface of oil but not on the surface of the water.
In this test, the emulsion is diluted either with oil or water. If the emulsion is o/w type and it is diluted with water, it will remain stable as water is the dispersion medium. For example, milk which is oil in water emulsion can be diluted with water, but butter which is water in oil emulsion cannot be diluted with water.
If emulsion o/w type is diluted with oil, the emulsion will break as oil and water are not miscible with each other.
Oil in water emulsion can easily be diluted with an aqueous solvent whereas water in oil emulsion can be diluted with an oily liquid.
Cobalt Chloride Test:
When a filter paper soaked in cobalt chloride solution is dipped into an emulsion and dried, it turns from blue to pink, indicating that the emulsion is o/w type.
If an emulsion on exposure to ultra-violet radiations shows continuous fluorescence under a microscope, then it is w/o type and if it shows only spotty fluorescence, then it is o/w type.
Properties of Emulsions:
In many emulsions, the size of the dispersed droplets is larger than the particles found in sols.
These droplets can be precipitated by adding suitable electrolytes.
Separation of two liquid phases (two distinct layers) from emulsions due to the removal of emulsifier by any means is called as de-emulsification or demulsification.
It can be done by heating, freezing, centrifuging or by addition of electrolytes.
Emulsions can be diluted with any amount of dispersion medium. But it is to be noted that any increase in the concentration of dispersed phase results in demulsification.
It is the phenomenon of disappearance of the boundary between two particles in contact (as in case of droplets and bubbles). The formation of aggregates may be followed by coalescence.
In excessive conditions, the coalescence leads to breaking of the emulsion.
Coalescence of solid particles is called sintering.
Emulsions show similar properties as that of sols. When the droplets are very small they show Tyndall effect, Brownian movement and electrophoresis.
Characteristics of Emulsions:
It is the colloidal system in which the dispersed phase and the dispersion medium both are liquid.
It is a mobile liquid.
Droplets of one liquid dispersed in another liquid.
No tendency to absorb a liquid or to swell.
The emulsifying agent is needed.
It is classified as oil in water and water in oil.
They show Tyndall effect, Brownian movement and electrophoresis.
Uses of Emulsions:
Phenyl, when poured in water, gives oil in water emulsion. It is used as a disinfectant.
Cleansing action of soap or detergent is due to emulsion formation of oil with water.
Emulsions are used in medicines such as emulsions of cod liver oil, malt and yeast.
They are used in oil paints, plastic emulsion paints.
In the concentration of sulphide ore by forth floatation method, oil in water type of emulsion is formed.
Milk is an important household emulsion.
Asphalt emulsified in water is used for the construction of roads.
Method of Breaking of Emulsions:
The two liquids in the emulsion can be separated by heating, freezing or centrifuging.
The addition of a large quantity of electrolyte causes coagulation of the dispersed phase.
The chemical destruction of the emulsifier causes the separation of the two liquids.
Disadvantages of Emulsion:
To refine the emulsion of petroleum with water is expensive.
The emulsion of oil in water may be coming as water supply, which is unfit for drinking.