A SIMULATION STUDY OF FLEXIBLE ZWITTERIONIC MONOLAYERS - INTERLAYER INTERACTION AND HEADGROUP CONFORMATION

A simple model for two opposing monolayers of flexible zwitterionic amphiphiles is studied by means of Monte Carlo simulation. The model allows for molecule fluctuations with a component normal to the average hydrocarbon/water interface arising from the partial diffusion of the hydrocarbon chains out of the hydrophobic core. For a range of model parameters we investigate two properties of the system: the headgroup conformation (orientation) and the separation dependence of the interlayer force. From a comparison with spectroscopic data we find that the average ''near-parallel'' orientation of the zwitterion dipole is as much as a consequence of the perpendicular fluctuations as of inter- and intralayer electrostatic interactions. The out-of-plane degree of freedom extends the range of a repulsive entropic contribution to the total interlayer force. The total force, which can be repulsive up to separations of 30 angstrom, is found to have an exponential dependence on surface separation. At larger separations attractive correlation contributions dominate, providing a limit to the ''swelling'' of the layers in excess water. The model produces trends qualitatively consistent with force measurements in systems of various phospholipids.