Statistical thermodynamics of association colloids. IV. Inhomogeneous membrane systems.

A previously developed self-consistent field theory to describe the equilibrium properties of lipid-like membranes is extended to allow the presence of inhomogeneities parallel to the membrane. Conformations of lipid molecules and foreign ('guest') molecules in the membrane are obtained by step-weighted random walk generation of the chains on a lattice, where the theory is made two-dimensional to account for the parallel inhomogeneities. No pre-assigned positions of the head groups, or other parts of the molecules, are introduced. Nearest neighbor interactions are accounted for through Flory-Huggins type interaction parameters. The theory is applied to the incorporation of trans-membrane molecules and conditions for channel formation are discussed. Lateral phase separation in membranes consisting of non-mixing lipids is also considered. Water only slightly enriches the boundary between the two lipid regions. The aliphatic chains are very well able to smoothly cover inhomogeneities in the bilayer. No indications of instability of the membrane due to the induced inhomogeneities are found.