Chemistry Reference
In-Depth Information
role of emulsifiers and stabilizers and their chemical structures, the role of the
chemical structures of the immiscible phases, and the effects of additives in each
phase. That represents a very tall order, as illustrated by the fact that even though
vast amounts of experimental data relating to each of those questions are available,
no generally applicable theory has yet appeared.
When discussing the stability of an emulsified system, it is important to have a
clear idea of the physical condition of the components and the terminology
employed. Four terms commonly encountered in emulsion science and technology
related to stability are ''breaking,'' ''coalescence,'' ''creaming,'' and ''flocculation.''
Although they are sometimes used almost interchangeably, those terms are in
fact quite distinct in meaning as far as the condition of an emulsion is concerned.
''Coalescence,'' for example, refers to the joining of two (or more) drops to form a
single drop of greater volume, but smaller interfacial area (Figure 9.1a). Such a
process is obviously energetically favorable in almost all cases. Although coales-
cence will result in significant microscopic changes in the condition of the dis-
persed phase, such as changes in average particle size and distribution, it may
not immediately result in a macroscopically apparent alteration of the system.
The ''breaking of an emulsion'' (Figure 9.1b) refers to a process in which a
gross separation of the two phases occurs. The process is a macroscopically appar-
ent consequence of the microscopic process of drop coalescence. In such an event,
the identity of individual drops is lost, along with the physical and chemical proper-
ties of the emulsion. Such a process obviously represents a true loss in the stability
of the emulsion.
''Flocculation'' refers to the mutual attachment of individual emulsion drops to
form flocs or loose assemblies of particles in which the identity of each is main-
tained (Figure 9.1c), 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. Finally, ''creaming'' is related to flocculation in that it occurs without the
Figure 9.1.
The ultimate fates of emulsions related to colloidal stability: (a) coalescence;
(b) breaking; (c) flocculation; (d) creaming.
