Phase Transition Behavior and Permeation Properties of Cationic and Anionic Artificial Lipids with Two Alkyl Chains

Membrane structure and crystal‐liquid crystal phase transition behaviors were investigated for the water system of totally artificial lipids with cationic or anionic head group and two alkyl chains. These artificial lipids formed multilamellar liposomes composed of a bimolecular layer. The phase transition temperature from gel to liquid crystalline lamellae decreased steadily to a limiting temperature with an increase in water concentration up to the lipid fraction of 0.80 — 0.85. This kind of phase diagram means that the artificial lipids have characteristics of both thermotropic and lyotropic mesomorphisms. For the cationic artificial lipid, the thickness of the bimolecular lamella in a gel state was shorter than a bimolecular length calculated on the assumption of planar zigzag conformation of the alkyl chains. This suggests that the hydrocarbon chains in a gel state are packed with the chain axes inclined at a certain angle to the lamellar surface. The diffusive permeability of the polymer/artificial lipid composite membranes to water or ions exhibits a distinct jump in the vicinity of the crystal‐liquid crystal phase transition temperature.