Van der Waals forces in oil/water systems

Abstract

Free energies of formation of “black” lipid films have been determined from their contact angles. In these films the repulsive forces are of such short range that it is possible to estimate relatively accurately that part of the free-energy change that originates from the van der Waals forces. It is shown that if the van der Waals free energies are interpreted on the assumption that the films are isotropic layers of hydrocarbon bounded by semi-infinite aqueous phases, the Hamaker coefficients vary consideraly from one film to another, contrary to the predictions of the Lifshitz theory. If, on the other hand, it is recognized that the hydrocarbon region of a film is, in fact, a layered structure and that there are differences, albeit small, between the dielectric properties of the chains of the lipid stabilizer and the alkane solvent (some of which is retained in the film), the conflict between theory and experiment is largely removed. Thus, Hamaker coefficients calculated from the Lifshitz theory for multilayered systems agree well in nearly all instances with the corresponding experimental coefficients.