On the image-charge model for the hydration force

Abstract

The image-charge model for the repulsive hydration force in multilamellar systems of zwitterionic molecules (e.g. phospholipid bilayers) proposed by Jönsson and Wennerström (J. Chem. Soc., Faraday Trans. 2, 1983, 79, 19) is analysed in some detail. Some over-restrictive assumptions in their model are removed and the hydration force is calculated in a greater variety of cases. In general, the force follows a power law. The leading term for large lamellar spacing is a repulsive, zwitterionic self-image interaction. The total force for reasonably small spacings depends crucially on the zwitterionic correlation within each bilayer and may be attractive as well as repulsive. The results are very dependent on the model and change dramatically when the location of the dielectric discontinuities between water and hydrocarbon relative to the polar headgroups is varied, unless the zwitterions lie flat parallel to the interface. If the lateral compressibility is (assumed) negligible and the zwitterions are perpendicular to the lamellar plane, the force law is exponential for a very particular kind of correlation function; this was the main case considered explicitly by Jönsson and Wennerström. The different force laws are compared with experimental values of the forces.