Planar bilayer lipid membranes, binary phase diagrams, and their relationship

Examination of a sequence of phase states in amphiphile/water systems has led to recognition of two of binary phase diagrams. The ability of substances to form the planar bilayer lipid membranes has shown to agree with a certain type of phase diagrams. The phase behaviour of membrane-forming amphiphiles was fairly independent of the content of water in the mixtures. Tn finding the reasons for the independence of phase states from amphiphile hydration, it has been ascertained that for these systems the dimensionless packing parameter, introduced by Israelachvili et al., is almost invariable with water added. The constancy is provided bf simultaneous, compensating changes in the polar region area and the hydrocarbon chain length of amphiphilic molecules. As a result, they retain their cylindrical shape over a wide range of water concentrations. These findings suggest that the propensity of lipids to adjust the molecular geometry themselves to short-term dehydration of their polar region, that accompanies cell-to-cell and membrane contacts or the action of some chemicals, and thus to keep the approximately cylindrical shape and unconstrained assembly, enables the lipid matrix to maintain the bilayer intact and provide the proper functioning of biological membranes. In case, the packing parameter is variable and dependent upon the water content in the mixtures, the tendency towards the formation of planar bilayer membranes has not been observed.