Van der Waals forces in oil-water systems from the study of thin lipid films - III. Comparison of experimental results with Hamaker coefficients calculated from Lifshitz theory

Abstract

From the Lifshitz theory, Hamaker coefficients have been computed for all the monoglyceride-alkane thin films in polar liquids described in the previous two parts of this paper. The calculations have been based on three different models of the films. The first model represents the film simply as an isotropic layer of liquid hydrocarbon, in which it is assumed that the lipid and solvent chains are intimately mixed. The second is similar to the first except that it includes two 0.4 nm layers of glycerol to represent the glyceride polar groups of the lipid. The theoretical Hamaker coefficients for these two models are identical to within ca . 10% and, for twelve out of the twenty-four systems examined, they agree to within ca . 20 % with the corresponding experimental coefficients. In the remaining systems, which mainly involve very thin films, the theoretical and experimental results differ by as much as a factor of three. In seeking an explanation for these discrepancies, calculations were carried out for the third model in which, instead of assuming complete mixing of the lipid and solvent chains, it is assumed that they are completely segregated. The hydrocarbon layer is thus split into three layers, i. e. lipid chains–alkane solvent–lipid chains. As with the first and second models, for about half the systems, theory and experiment agree to within 20 % but, for the remaining films, the third model is a considerable improvement as it yields no discrepancies with the experimental data greater than a factor of two. Such discrepancies as do still exist may be accounted for by recognizing that some solvent must in­evitably be present in the lipid chain regions and that, in fact, the state of the interior of a film is intermediate to those assumed in the second and third models. The results exemplify well how successful the Lifshitz theory can be in liquid systems, but they also emphasize the very large effects which small differences in dielectric properties can have on the van der Waals forces in thin films.