The Smectic Phase of Lipid-Water Systems: Properties Related to the Nature of the Lipid and to the Presence of Net Electrical Charges

The lamellar phase of the lipid-water systems, when studied as a function of the water content, displays properties that belong to one of the following classes, according to the nature of the lipid. (a) The lamellar phase takes up an almost indefinite amount of water intercalated between the lipid layers (lipids bearing net electrical charges). (b) The maximum amount of intercalated water is rather small, and the water in excess stays as a separate phase (lipids devoid of net charges). (c) The amount of water in the lamellar phase is limited: further addition of water induces a transition to another liquid-crystalline phase (soaps and detergents). It is shown here that if small amounts of ionizable lipids, either anionic or cationic, are incorporated to a non-ionizable lipid, the lamellar phase takes up very large amounts of water, EW in the lipids of class (a): this phenomenon is not observed if the added lipids are an equinormal mixture of anionic and cationic species. These experiments stress the rôle of the electrical charges as stabilizing factor in the lipid-water phases. The rôle of this and other factors is discussed.