7.2 Physical Nature of the Interfacial Film

The droplets of dispersed liquid in an emulsion are in constant motion and therefore there are frequent collisions between them. If，on collision，the interfacial film surrounding the two colliding droplets in a macroemulsion ruptures，the two droplets will coalesce to form a large one，since this results in a decrease in the free energy of the system. If this process continues，the dispersed phase will separate from the emulsion，and it will break. The mechanical strength of the interfacial film is therefore one of the prime factors determining macroemulsion stability. For maximum mechanical stability，the interfacial film resulting from the adsorbed surfactants should be condensed，with strong lateral intermolecular forces，and should exhibit high film elasticity.

Since highly purified surfactants generally produce interfacial films that are not close packed and hence not mechanically strong，good emulsifying agents are usually a mixture of two or more surfactants rather than an individual surfactant. A commonly used combination consists of a water-soluble surfactant and an oil-soluble one.