Induction of a hexagonal phase in phospholipid-surfactant bilayers

The possibility of modifying the curvature of lipid bilayers by mixing them with additives is demonstrated and the evolution of geometrical parameters with composition is discussed. X-ray diffraction patterns of the POPC/C₁₂EO₂/²H₂O system were observed as a function of the relative humidity. The formation of an unexpected hexagonal phase indicates peculiar behaviour in these mixtures. The cylinder radius of this phase is considerably smaller than previously observed in cubic phases. A discussion of the head group interactions is presented. We have also been able to show that the uptake of water by the L _β gel phase is higher as a single phase than in the L _β +H_II two phase region. The water content is important for the stabilization of H_II phase and determination of its characteristic dimensions. However, it is argued that the interaction between the surfactant and the lipid is the key factor for its formation and that the EO₂ head groups displace water from the inner parts of the polar region of the mesogenic units. Received: 22 September 1997 / Revised version: 31 December 1997 / Accepted: 10 February 1998