Diffusion of univalent ions across the lamellae of swollen phospholipids

The diffusion of univalent cations and anions out of spontaneously formed liquid crystals of lecithin is remarkably similar to the diffusion of such ions across biological membranes. If the unit structure of the liquid crystal is accepted as being that of a bimolecular leaflet, then these leaflets have been shown to be many orders of magnitude more permeable to anions than to cations. The diffusion rate for cations is very significantly controlled by the sign and magnitude of the surface charge at the water/lipid interface. There is a decrease of the diffusion rate for cations as the negative charge on the lipid structure decreases—which diminishes to zero for a positively charged membrane—the diffusion rate of anions remaining very high. The exchange diffusion rate of Cl− and I− was greater than that of F−, NO3−, SO42− and HPO42− but no significant differences were detectable for the cation series, Li+, Na+, K+, Rb+ and choline. The membranes are very permeable to water. Because the diffusion rate of cations is low, the phospholipid liquid crystalline structures appear to “bind” or “capture” cations. It is found that as the surface charge of the lipid lamellae is increased, the amount of cation per μmle of lipid increases. It is argued that if the cation is sequestered in aqueous compartments between the bimolecular leaflets, and if the thickness of the aqueous compartments is determined by the surface charge density of the lipid head groups and by the ionic strength of the aqueous phase in accordance with double-layer theory, the amount of cation trapped would also be expected to vary.