Toward the correct calculation of van der Waals interactions between lyophobic colloids in an aqueous medium

Abstract

Abstract We have developed an accurate and simple method for calculating van der Waals (vdw) forces in condensed media based on the Lifshitz theory and have generalized that theory to apply to the interaction of two planar parallel lipid slabs in an aqueous medium. Several features of vdw forces in lipid-water systems are qualitatively different from the common assumptions of colloid science: (1) vdw energies are not the sum of ( λ / r 6 ) interactions between small elements of the interacting bodies; (2) electromagnetic fluctuations at ultraviolet frequencies, usually considered exclusively, are relatively unimportant compared to fluctuations at infrared and microwave frequencies; (3) by virtue of the strong microwave contribution, vdw forces in lipid-water mixtures increase with temperature and exhibit only weak “retardation” effects. Available spectral information is sufficient to derive these conclusions with little ambiguity. In particular, the feature of nonadditivity (which makes even the algebraic form of the interaction different from that expected) depends primarily on dielectric constants of component materials in the limit of zero frequency. Lifshitz' approach is conceptually clear. It correctly accounts for all frequencies of interaction, implicitly includes all many-body effects, and all consequences of intermediate substances. There is now good reason to use it in practice to calculate vdw forces for colloids at large separation distances.