General Considerations of Emulsion Formation and Stability

The question of emulsion stability has already been raised in the context of emulsion preparation. However, the preparation process is very dynamic and represents a complicated combination of events that is not easily analyzed. Once prepared, however, and left at rest to do its own thing, the fates of individual droplets become more readily determined and some semblance of understanding can be extracted from the initial chaos. That is not to say, however, that there exists a good general theory of emulsion stability that one can apply under all, or even most, circumstances. 

Even though emulsions as defined have been in use for thousands of years (even longer if natural emulsions are considered), no comprehensive theory of emulsion formation and stabilization has yet been developed that adequately describes, and predicts, the characteristics of many of the complex formulations encountered in practice. Except in very limited and specialized areas, the accurate prediction of such aspects of emulsion technology as droplet size, size distribution, and stability remain more in the realm of art than true science. 

The breaking of an emulsion (Fig. 112b) refers to a process in which a gross separation of the two phases occurs. The process is a macroscopically 

FIGURE 11.2. An emulsion may have several fates depending on the system and conditions. The droplets may coalesce to form larger drops (a). All drops may coalesce to form two separated phases b). Droplets may flocculate to form larger aggregates while maintaining their individuality (c). Flocculated droplets may produce two apparently separate phases one rich in dispersed phase, but with individual droplet identity, and one rich in continuous phase with few or no droplets present (d). 

Between the two clearly distinguishable states of an emulsion lies flocculation, which refers to the mutual attachment of individual emulsion drops to form floes or loose assemblies of particles in which the identity of each is maintained (Fig. 11.2c), a condition that clearly differentiates it from the action of coalescence. Flocculation can be, in many cases, a reversible process, overcome by the input of much less energy than was required in the original emulsification process. 

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