CURVATURE ELASTICITY AND THERMODYNAMIC STABILITY OF ELECTRICALLY CHARGED MEMBRANES

The elastic behavior and stability of electrically charged amphiphilic membranes is investigated. In the present study, we address the quest ion whether the electrostatic contribution to the curvature elastic mo duli of a uniformly charged symmetric membrane leads to a curvature in stability. To this end we consider a membrane in which the overall num ber of molecules is conserved during any deformation. In order to esti mate both the molecular equilibrium area and the position of the neutr al surface of each monolayer during bending, we include in the express ion of the bilayer free energy beside an electrostatic, also a nonelec trostatic contribution. The former is described within the Gouy-Chapma n theory of the diffuse double layer. The latter is a sum of a chain, an interfacial, and a nonelectrostatic head group contribution. The ch ain part is described using a detailed mean-field conformational free energy which is based on a molecular chain model. For the interfacial and nonelectrostatic head group contribution we use simple but general phenomenological expressions. It is shown that for medium and high me mbrane surface charge densities the electrostatic contribution to the bending moduli is not negligible. For highly charged membranes, the mo del predicts an instability with respect to a spherical deformation. T his is discussed referring to the experimentally observed process of s pontaneous vesiculation upon jump in pH of certain ionizable amphiphil ic molecules. (C) 1996 American Institute of Physics.