DEFECTS AND THE SOL-GEL TRANSITION IN LAMELLAR FLUID MEMBRANES

Phase transitions of a multilamellar system L-alpha composed of water, lipids and PEG-lipids are investigated. Increasing the water fraction or the PEG-lipids concentration of such a system, the fluid multilame llar phase changes to a gel phase and then returns to a fluid phase. S ince flat layers with a sufficient concentration of PEG-lipids are uns table with respect to undulation fluctuations, we assume that undulati ons of large amplitude create certain kinds of defects, which are comp osed of folded membranes and mulitilayer spherical or cylindrical vesi cles. The concentration of these defects changes the viscosity of the system. We calculate the concentration of defects using an ideal gas a pproximation and show that the transition point from the fluid to gel phase is inversely proportional to the membrane separation, under the assumption that the energetically favorable defect size is independent of the separation. We predict three types of sol-gel transitions, whi ch qualitatively explain the phase diagram obtained by experiments.