Fluid lipid bilayers: Intermonolayer coupling and its thermodynamic manifestations

A fluid membrane of lipid bilayer consists of two individual molecular monolayers physically opposed to each other. This unique molecular architecture naturally necessitates the need to treat a lipid-bilayer membrane as one entity of two coupled two-dimensional systems (monolayers), each of which possesses “in-plane” degrees of freedom that characterize its physical or chemical state. Thermally excitable deformations of a lipid bilayer in its geometrical conformation further impart to it “out-of-plane” degrees of freedom. In this paper we discuss the issue of intermonolayer coupling in terms of a phenomenological model that describes the necessary types of degrees of freedom and their interplay, which reflects different modes of intermonolayer coupling. Furthermore, we investigate, based on the phenomenological model, the manifestations of the intermonolayer coupling both in the lateral ordering processes of the “in-plane” degrees of freedom and in the conformational behavior of the bilayer membrane.