The role of surface films in emulsification

The role of surface films in emulsification.111 Emulsions, or dispersions of one liquid in another, are of two types. One consists of a very dilute dispersion of small droplets of oil in water, in the stabilization 

The more common type of emulsion depends largely for its stability on the nature of the interfacial film, generally of a third substance different from either phase, but slightly soluble in one of them, present between the two liquids when one is dispersed in the other. One of the principal functions of the interfacial film is to decrease the interfacial tension, by increasing the adhesion between the two phases, and thus lowering the amount of work that must be done in creating the (often very large) area of interface between the liquids, as one is dispersed in the other. 

The reason why a closely packed film of stabilizer, such as soap, is more effective than a loosely packed one, is probably simply that when the packing is loose, the molecules may be pushed out of the way, away from the parts of the surfaces of two droplets which approach nearest when two molecules are about to collide. Assuming, as is probably the case, at 

du Nouy, Surface Equilibria of Colloids (Chem. Catalog Co., 1926). 

Harkins s results appear to show that the stabilizing film of soap is never thicker than monomolecular. 

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