Fundamental Concepts in Emulsion Science and Technology

As is the case in most discussions of interfacial systems and their applications, definitions and nomenclature can play a significant role in the way the material is presented. The definition of an emulsion to be followed here is that they are heterogeneous mixtures of at least one immiscible liquid dispersed in another in the form of droplets, the diameters of which are, in general, greater than 0.1 (.m. Such systems possess a minimal stability, generally defined rather arbitrarily by the application of some relevant reference system such as time to phase separation or some related phenomenon. Stability may be, and usually is, enhanced by the inclusion of additives such as surfactants, finely divided solids, and polymers. Such a definition excludes foams and sols from classification as emulsions, although it is possible that systems prepared as emulsions may, at some subsequent time, become dispersions of solid particles or foams. 

When discussing emulsions, it is always necessary to specify the role of each of the immiscible phases of the system. Since in almost all cases, at least one liquid will be water or an aqueous solution, it is common practice to 

There is, in principle, no reason why one cannot prepare an oil-in-oil emulsion (o/o). However, the generally high miscibility of most organic liquids is an important limitation. More important, however, is the fact that the nature of interfaces dictates that a system tends to attain a situation of minimum energy, in this case minimum interfacial area, so that some additive must be employed to retard that process. Unfortunately, few materials are sufficiently surface active at such oil-oil interfaces to impart the required minimal stability necessary for the preparation and maintenance of such emulsions. Oil-in-oil emulsions of short persistence can, however, constitute an intermediate step in the preparation of nonaqueous emulsion polymers. 

Three major characteristics of an emulsion must be considered  

Which of the two liquid phases will be the continuous phase and which the dispersed phase when the emulsion is formed, and what factors can be used to control that result. 

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