Coalescence as the Result of Hole Formation

In many emulsions in which coalescence is observed, one would not expect it to occur from an examination of the interdroplet pair potential, because of the extremely high steric repulsion that arises when two droplets closely approach each another (Section 3.5). The interdroplet pair potential, which describes the dependence of the colloidal interactions between the emulsion droplets on separation, assumes that the system is at thermodynamic equilibrium. It therefore predicts that droplets will remain indefinitely in a potential energy minimum. In practice, the stability of an emulsion to coalescence is often governed by nonequilibrium processes associated with the dynamic events that occur on the molecular level (Evans and Wennerstrom 1994, Kabalnov and Wennerstrom 1996).

It is widely accepted that coalescence occurs as the result of the formation of a hole which extends across the interfacial membranes surrounding the droplets (Figure 7.17). Once the hole is formed, the liquid in the droplets rapidly flows through it and the droplets merge together to form a single larger droplet. The coalescence rate therefore depends on the likelihood that these holes will form in the interfacial membranes.

Holes may be formed in an interfacial membrane in a variety of different ways, depending on type of emulsifier used and the environmental conditions. They form spontaneously in surfactant monolayers due to thermal fluctuations of their shape (Section 7.5.1.4). They may also be formed as the result of the chemical breakdown of an emulsifier over time (e.g., a protein or polysaccharide may be cleaved into smaller fragments due to enzymic or chemical hydrolysis). These fragments might not be surface active or they may not form a good protective membrane, which leads to the formation of a hole. A hole may also be created when emulsifiers are displaced from the surface of emulsion droplets by more surface-active components which do not form membranes resistant to rupture (e.g., biopolymers may be displaced by high concentrations of alcohol). Which of these mechanisms is important in a particular system depends on the composition and microstructure of the emulsion, as well as environmental conditions such as pH, ionic strength, ingredient interactions, temperature, and mechanical agitation (Section 7.5.3). Quantitative predictions of the rate at which coalescence proceeds in real emulsions are extremely difficult because of the complex nature of the processes involved, the difficulty in developing theories to describe these processes, and the lack of experimental techniques to provide the information required to test these theories. The

FIGURE 7.17 Coalescence of droplets depends on (a) film thinning and (b) film rupture. (a) As two droplets approach each other, the liquid between them gets increasingly thinner and may continue to thin or may reach some equilibrium value. (b) Two droplets that are in contact may spontaneously merge because of the thermal fluctuation of their membranes, leading to hole formation.

FIGURE 7.17 Coalescence of droplets depends on (a) film thinning and (b) film rupture. (a) As two droplets approach each other, the liquid between them gets increasingly thinner and may continue to thin or may reach some equilibrium value. (b) Two droplets that are in contact may spontaneously merge because of the thermal fluctuation of their membranes, leading to hole formation.

most significant progress in this area has been made in understanding the origins of hole formation and coalescence in emulsions stabilized by small-molecule surfactants (see next section).

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