0/W Unstable W/0

FIGURE 4.15 The phase inversion temperature occurs when the optimum curvature of a surfactant monolayer is zero.

is known as the phase inversion temperature or PIT (Shinoda and Kunieda 1983, Shinoda and Friberg 1986). Consider what happens when an emulsion that is stabilized by a surfactant is heated (Figure 4.15). At temperatures well below the PIT (~20°C), the packing parameter is significantly less than unity, and so a system that consists of an oil-in-water emulsion in equilibrium with a swollen micellar solution is favored. As the temperature is raised, the hydrophilic head groups of the surfactant molecules become progressively dehydrated, which causes p to increase toward unity. Thus the emulsion droplets become more prone to coalescence and the swollen micelles grow in size. At the PIT, p = 1, and the emulsion breaks down because the droplets have an ultralow interfacial tension and therefore readily coalesce with each other (Aveyard et al. 1990, Kabalnov and Weers 1996). The resulting system consists of excess oil and excess water (containing some surfactant monomers), separated by a third phase that contains surfactant molecules aggregated into bilayer structures. At temperatures sufficiently greater than the PIT (~20°C), the packing parameter is much larger than unity, and the formation of a system which consists of a water-in-oil emulsion in equilibrium with swollen reverse micelles is favored. A further increase in temperature leads to a decrease in the size of the reverse micelles and in the amount of water solubilized within them. The method of categorizing surfactant molecules according to their molecular geometry is now widely accepted as the most useful means of determining the type of emulsions they tend to stabilize (Kabalnov and Wennerstrom 1996).

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