FIGURE 7.18 Coalescence may occur when the optimum curvature of a surfactant is similar to that of the edge of the hole.

hand, if the optimum curvature of the surfactant is opposite to that of the edge, then the formation of a hole is energetically unfavorable. The dependence of hole formation on the optimum curvature of a membrane means that coalescence is related to the molecular geometry of the surfactant molecules (Section 4.5). The relationship between the molecular geometry and coalescence stability of oil-in-water and water-in-oil emulsions is highlighted in Figure 7.18. As with the other coalescence mechanism described above, hole formation depends on thermal fluctuations of the interfacial membrane so that it is able to change its curvature. The coalescence rate will therefore increase as the interfacial tension and rigidity of the membrane decrease.

An additional factor which contributes to the stability of surfactant-stabilized emulsions in which collision-induced coalescence is important is the Gibbs-Marangoni effect (Walstra 1993b, 1996a,b). As two droplets approach each other, the liquid in the continuous phase is forced out of the narrow gap which separates them. As the liquid is squeezed out, it drags some of the surfactant molecules along the droplet surface, which leads to the formation of a region where the surfactant concentration on the surfaces of the two emulsion droplets is lowered (Figure 7.19). This causes a surface tension gradient at the interface, which is energetically unfavorable. The surfactant molecules therefore have a tendency to flow toward the region of low surfactant concentration and high interfacial tension, dragging some of the liquid in the surrounding continuous phase along with them. This motion of the continuous

Low y High y Low y

Low y High y Low y

Surfactcm Surfactan

FIGURE 7.19 Gibbs-Marangoni effect.

phase is in the opposite direction of the outward flow that occurs when it is squeezed from between the droplets, and therefore it opposes the movement of the droplets toward each other and therefore increases their stability to coalescence.

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