Emulsions, Foams, and Aerosols

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Microemulsions are treated in a separate section in this chapter. Unlike macro- or ordinary emulsions, microemulsions are generally thermodynamically stable. They constitute a distinctive type of phase, of structure unlike ordinary homogeneous bulk phases, and their study has been a source of fascination. Finally, aerosols are discussed briefly in this chapter, although the topic has major differences from those of emulsions and foams. 

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Suspensions, and to some degree emulsions, foams and aerosols, play crucial roles in the evolution of the earth s rocks, rivers, streams, lakes, oceans and soils. Table 9.1 lists some examples. In many cases, their role is somewhat disguised in that these colloidal dispersions are the precursors to the ultimate products, the latter having very different final appearances, such as many rocks, sediments and soils. 

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