Statistical thermodynamics of association colloids. I. Lipid bilayer membranes

Abstract

Step-weighted random walks (modified Markov chain statistics) combined with a self-consistent-field approximation form the basic concepts of a Flory-Huggins-type of theory to describe the lipid bilayer.1 The purpose of the present paper is to extend this model by incorporating the rotational isomeric state scheme, both for linear and branched chain molecules. Only three measurable interaction energy parameters of a Flory-Huggins-type are required, namely for the head group tail, the head group water, and the tail water contacts. In addition, the theory needs one energy parameter for the internal trans/gauche transition energy of the chain. Results of this self-consistent-field (SCF) theory are given for membranes formed by lecithin-like molecules. With respect to earlier work, more detailed insight is obtained in the behavior of the lipid bilayer above the gel to liquid phase transition temperature. Equilibrium conditions are formulated. Segment density profiles and solvent distributions are calculated. It is shown that the two apolar tails of the lecithin do not behave identically. The tail next to the head group is lifted slightly more out of the membrane than the other tail. The well-known balance of forces, responsible for membrane formation is analyzed. We found that the repulsive tail head interaction, often ignored in theories, is essential for the stability of association colloids. © 1988 American Institute of Physics.