Van der Waals forces in oil—water systems from the study of thin lipid films - II. The dependence of the van der Waals free energy of thinning on film composition and structure

Abstract
The van der Waals contribution to the free energy of thinning of lipid films has been determined for a variety of lipids, solvents and polar phases, and Hamaker coefficients have been calculated from this free energy, using the equation for a simple three layer (water-hydrocarbon-water) model. The results confirm and extend the findings presented in part I (Requena, Billett & Haydon 1975) i.e. that, for systems of comparable composition, the Hamaker coefficient approaches a constant value as the film thickness increases but that, towards smaller thicknesses, the coefficient tends to rise rapidly. This latter observation is not readily accounted for in terms of the three layer structure, and experiments have been carried out in order to pinpoint the shortcomings of the model. Interaction between the layers of lipid polar groups is shown to have been justifiably neglected, but it is inferred that small variations across the films of the dielectric permeability of the hydrocarbon may play a considerable part in determining the Hamaker coefficient.

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