Surface Films of Insoluble Substrates

Let us turn away briefly from the solution properties and bulk phase behavior of surfaetants. Much has been learned about the properties of surfactant molecules at interfaces by the study of molecules with hydrocarbon chains so long that they are virtually insoluble in water. 

Film balance data are normally presented as plots of the surface pressure as a function of the area o per molecule. The latter is obtained by dividing the 

FIGURE 4.19 Schematic diagram of a Langmuir trough. The monolayer is deposited to the right of the barrier, and the barrier can be moved across the surface to change the area accessible to the monolayer. The surface pressure can be measured either by determining the force on a float that separates the monolayer from a clean water surface, or from the difference in the force exerted on the Wilhelmy plate when the plate is suspended in pure water and in water covered by the monolayer. From Knobler (1990), this material is used by permission of John Wiley Sons, Inc. 

As indicated in Chapter 2, this equation can be modified to account for nonideal effects at smaller values of o by methods similar to those used for three-dimensional equations of state for nonideal gases. 

The Brewster angle microscope developed by Henon and Meunier (1991) does not require addition of a probe and also yields information on the morphology of surface phases. VoUhardt (1996) reviewed results obtained with this 

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