Emulsification is the method of dispersing a liquid in another immiscible liquid, The major groups of emulsifying agents are soaps, detergents, and compounds whose basic structure is a paraffin chain ending in a polar group.

Bancroft stated that the essential conditions for a stable emulsion are that the drops of the dispersed phase should be small enough to remain suspended and a suitably viscous film should be present around each drop of the dispersed phase to prevent them from combining.

Many theories have been introduced to describe the way through which the emulsions are stabilised by the emulsifier. Presently, no such theory has been proposed which can be universally applied to all emulsions. Fischer suggested that a few factors play a role, and the importance of each varies in different emulsions and also in the same emulsion under different conditions.

Electrical Double Layer Theory

Negatively charged oil globules are present in a pure oil and pure water emulsion. Water ionises into hydrogen and hydroxyl ions. The adsorption of hydroxyl ions gives the negative charge on the oil globules. These adsorbed hydroxyl ions layer around the oil globules.

The oppositely charged ions in the liquid form another layer around the layer of negative ions. Thus, two layers of oppositely charged ions are formed, known as the Helmholtz double layer. This layer is not limited to emulsions but is seen in all the boundary phenomena. The electric charge is a factor in all emulsions, even those stabilised with emulsifying agents.

Phase-Volume Theory

On closely packing some spheres of same diameter, one sphere will touch the other spheres and they together occupy about 74% of the total volume. Thus, if the spheres or droplets of the dispersed phase remain rigid, 74 parts of the dispersed phase can be dispersed in the continuous phase; on the contrary if the dispersed phase is increased to more than 74 parts of the total volume, a reversal of emulsion occurs. The dispersed phase, however, does not remain rigid but the drops become flat and come in contact with each other.

Hydration Theory of Emulsion

Fischer and Hooker stated that hydrated colloids are the best emulsifying agents. Fischer stated that the emulsifying agents act as hydrophilic colloids in case of water and oil emulsions and they act as lyophilic colloids in case of non-aqueous mixtures. Thus, oil cannot be permanently beaten into water, but only to a colloid hydrate.

Fischer and Hooker found albumin, casein, and gelatin as good emulsifying agents. Non-hydrated casein at its isoelectric point acts as a poor emulsifying agent, while hydrated casein (acid or alkali casein) acts as a good emulsifying agent.

Fischer stated that the permanent emulsions can be explained on the basis of hydrated or lyated colloids. He also stated that conversion of water changes into a colloid hydrate, also changes its physical constants (such as surface tension, viscosity, and adsorption). Colloid treatment by freezing, heating, or adding substances which vary the water-holding capacity of the colloid, either crack the emulsion or reduces its ability to emulsify.

Oriented Wedge Theory

Langmuir and Harkins postulated this theory to state the manner in which emulsions are stabilised. This theory is based on the concept that the molecules of emulsifying agents orient themselves at the interface between the dispersed and continuous phases, to form a wedge. The curvature of this wedge determines the size of the dispersed phase.

Adsorbed Film and Interfacial Tension Theory

This theory is a modification of the previous theories. Currently, it is the most universally accepted theory for emulsion formation.

The following parameters influence the process of emulsification:

1. The mass of emulsifying agent,
2. The ease with which the emulsifying agent gets adsorbed at the interfacial separating surface, and
3. The nature of the ions adsorbed at the resultant film.

The emulsifying agent gets adsorbed by the water or oil. But, it gets adsorbed more in one liquid than in the other, thus lowers the interfacial tension of one liquid to a greater extent than the other. If the tension of water is reduced more than that of the oil, the tendency of water to form droplets is lowered, the water flows to form a film, and becomes the continuous phase.

Thus, the type of emulsion formed depends on the nature of emulsifying agent. If the solubility of emulsifying agent is more in water than in the oil, the water becomes the continuous phase; and if the wettability of emulsifying agent is more by water than by the oil, the water becomes the continuous phase. An emulsion is not formed if same tension exists on each side of the film or the emulsifying agent. This occurs when the mixture contains opposing emulsifying agents and each counterbalances the effect of other.

An emulsion can be formed when the emulsifying agent gets adsorbed at the interfacial surface and form a coherent film to stabilise the emulsion. The reversal of emulsion depends on the nature of ions adsorbed by this film.