Structural phase transitions of interacting membranes

Abstract

A thermodynamic model for phase transitions of multilamellar liquid crystals composed of phospholipid bilayers in water is developed. By means of a Landau theory of the thermally driven hydrocarbon chain-melting transition of an isolated membrane and a continuum model of the interactions between neutral phospholipid bilayers, the phenomenon of structural phase transitions induced by membrane interactions is described. The phenomenological parameters that appear in the theory may be readily determined by existing experimental techniques; an analysis of available data on several phospholipid systems is used here to estimate these quantities. Semiquantitative agreement is achieved with the experimental temperature-composition phase diagram as well as the variation of the characteristic dimensions of the lamellar crystals with water content. Possible origins of pseudocritical phenomena are suggested on the basis of the unusual topology of the phase diagram. The model highlights the central importance of the water-mediated ‘‘hydration repulsion’’ between membranes in the phase behavior of the lamellar systems.