Forces between phospholipid bilayers

Abstract

The measurement of forces between lipid bilayers is complicated by the coupling which exists between bilayer structure and interbilayer interaction. In fact, direct measurements are impracticable and methods must be sought which enable these two factors to be separated. A procedure is developed for calculating the bilayer stress, γ, and the interbilayer force, ƒ, from simultaneous X-ray measurements of the bilayer structure and water layer thickness and the activity of water. The method is applied to existing experimental measurements for the egg-lecithin–water system. The interbilayer ‘hydration force’ is found to be attractive, exponential with a decay length of 0.22 nm and to dominate completely the van der Waals dispersion force for bilayer separations well beyond the swelling limit. The spontaneous uptake of water by the lamellar phase is shown to be a consequence of the decrease in Gibbs energy associated with the lateral expansion of the bilayer. The elastic area compressibility modulus, k_a, near to the swelling limits is found to be 140 mN m^–1. The implications of these results for the phase behaviour of phospholipid–water mixtures and the stability of vesicle dispersions are discussed.