Gas Liquid Interface in the Presence of an Emulsifier

The concept of the Gibbs dividing surface is particularly useful for defining the amount of emulsifier which accumulates at an interface (Hunter 1986). Consider a system which consists of a surfactant solution in contact with its vapor (Figure 5.6). The emulsifier is distributed among the bulk aqueous phase, the vapor, and the interfacial region. The excess

Liqiud Gas Interface
FIGURE 5.5 From a thermodynamic standpoint, it is convenient to locate the interface (Gibbs dividing surface) separating a liquid and its vapor where cexcess = cdeflcit.

emulsifier concentration at the surface (n) corresponds to the total amount of emulsifier present in the system minus that which would be present if the emulsifier were not surface active and equals the shaded area shown in Figure 5.6. The accumulation of emulsifier molecules at an interface is characterized by a surface excess concentration (r), which is equal to the excess emulsifier concentration divided by the surface area: r = nJA. Food emulsifiers typically have r values of a few milligrams per meter squared (Dickinson 1992, Dalgleish 1996b). It is important to note that the emulsifier molecules are not actually concentrated at the Gibbs dividing surface (which is infinitely thin), because of their finite size and the possibility of multilayer formation. Nevertheless, this approach is extremely convenient for thermodynamic descriptions of the properties of surfaces and interfaces (Hunter 1986). The surface excess concentration is often identified with an experimentally measurable parameter called the surface load, which is the amount of emulsifier adsorbed to the surface of emulsion droplets per unit area of interface.

Gibbs Dividing Surface
FIGURE 5.6 When an emulsifier is present, the Gibbs dividing surface is conveniently located at the position where cexcess = cdeficit for the liquid in which the emulsifier is most soluble, and the surface excess concentration is equal to the shaded region.

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