The interface that separates the oil and water phases is often assumed to be a planar surface of infinitesimal thickness (Figure 5.1a). This assumption is convenient for many purposes, but it ignores the highly dynamic nature of the interfacial region, as well as the structure and organization of the various types of molecules involved (Figure 5.1b). On the molecular level, the oil and water molecules intermingle with each other over distances of the order of a few molecular diameters (Everett 1988, Evans and Wennerstrom 1994). The composition of the system therefore varies smoothly across the interfacial region (Figure 5.1b), rather than changing abruptly (Figure 5.1a). The thickness and dynamics of the interfacial region depend on the relative magnitude of the interactions between the molecules involved (oil-oil, water-water, and water-oil), which can be characterized by the effective interaction parameter (w) discussed in Section 2.6.2 (Evans and Wennerstrom 1994). If this parameter is positive and much larger than the thermal energy (w/kT >> 1), the interactions between the different types
of molecules are highly unfavorable and the interfacial region is relatively thin because the protrusion of a molecule from one phase into another involves a large expenditure of energy. If the effective interaction parameter is approximately equal to the thermal energy (w/kT ~ 1), the liquids are partially miscible and the interfacial region increases in thickness. When the effective interaction parameter is negative and much smaller than the thermal energy (w/kT << 1), the two liquids are miscible and the interfacial region disappears altogether. The thickness and mobility of the interfacial region are therefore governed by a balance between the interaction energies and the thermal energy of the system (Everett 1988, Israelachvili 1992).
Oil molecules are incapable of forming hydrogen bonds with water molecules, and so the mixing of oil and water is strongly unfavorable because of the hydrophobic effect (Section 4.4.3). It is therefore necessary to supply energy to the system in order to increase the contact area between oil and water molecules. The amount of energy which must be supplied is proportional to the increase in contact area between the oil and water molecules (Hiemenz 1986):
where AG is the free energy required to increase the contact area between the two immiscible liquids by AA (at constant temperature and pressure), and y. is a constant of proportionality called the interfacial tension. If one of the phases is a gas, the interfacial tension is replaced by the surface tension (ys). The interfacial tension is a physical characteristic of a system which is determined by the imbalance of molecular forces across an interface: the greater the interfacial tension, the greater the imbalance of forces (Israelachvili 1992, Evans and Wennerstrom 1994). Many of the most important macroscopic properties of food emulsions are governed by this imbalance of molecular forces at an interface, including the tendency for droplets to be spherical, the surface activity of emulsifiers, the nucleation and growth of ice and fat crystals, meniscus formation, and the rise of liquids in a capillary tube (Section 5.13).
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