INTERBILAYER REPULSION IN NONIONIC SURFACTANT WATER LAMELLAR PHASES .1. A MONTE-CARLO STUDY OF A DIPOLE ARRAY MODEL WITH DISCRETE WATER-MOLECULES

We report a simulation study using the RIB modification of the Gibbs' ensemble, for a model membrane system in which the surfaces have embedded dipoles, and are separated by a fluid of discrete TIPS2 water molecules. The separation between the layers is allowed to vary, as is the number of water molecules in the membrane which is in equilibrium with a vapour phase, the whole system being kept at constant (N, V, T). The simulations were performed at 298 K and two systems were studied: in the first, the surface dipoles were modelled as OH groups attached to hydrocarbon chains; in the second, the charge on the O and H atoms was arbitrarily doubled to 0.734e. The structure of the water around the surface dipoles was examined by collecting density and orientational distributions for different regions in the plane of the surface and for molecularly thick layers of water. Although local water structure is modified by the surface charges, this did not result in a stable bilayer for either of the two dipole strengths. The study has relevance to the stability of nonionic surfactants and phospholipids, and suggests that a factor additional to surface charge is needed to explain the stable structures observed experimentally.