Classification of Emulsifiers and Stabilizers

There are four general classes of materials that can, under the proper circumstances, act as emulsifiers and/or stabilizers for emulsions. The list includes 

Small colloid materials (sols), while not directly affecting interfacial tensions, can stabiUze an emulsion by forming a physical barrier between drops, thereby retarding or preventing drop coalescence (Fig. 11.lb). The action of such materials will depend on several factors, the most important of which 

FIGURE 11.1. There are four primary mechanisms for the stabilization of emulsions (plus combinations, of course). Some emulsions may be weakly stabilized by the presence of adsorbed ions and nonsurface-active salts (a). The presence of colloidal sols partially wetted by both phases of the emulsion may form a mechanical barrier to drop contact and coalescence (b). Many emulsions are stabilized by adsorbed polymer molecules (c). Along with polymers, adsorbed surfactant molecules represent the most common stabilization mechanism (d). 

Polymeric additives may aid in emulsion formation as a result of surface-active properties but are usually more important as stabihzers. Their action may result from steric or electrostatic interactions, from changes in the interfacial viscosity or elasticity, or from changes in the bulk viscosity of the system. In many if not most cases, the function of polymeric stabihzers is a combination of several actions (Fig.ll.lc). 

FinaUy, normal monomeric surfactants are usuaUy added in order to decrease the interfacial tension and impart added stabihty to the system (Fig. ll.ld). The type and quantity of surfactant employed will be determined by the specific properties of the liquid phases, the type of emulsion desired, conditions of use, and other factors. 

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