A lattice model of a supported monolayer of amphiphile molecules: Monte Carlo simulations

We describe a lattice model of a supported monolayer of flexible amphiphile molecules which treats their full three-dimensional structure and includes nearest neighbor interactions between segments of the amphiphile chains, an attraction between the chain segments and the underlying substrate, and the inter- and intrachain excluded volume. The thermodynamic and conformational properties of this system are determined by a Monte Carlo simulation technique using a Metropolis sampling. At low surface coverage, the thermodynamic behavior predicted by the simulations is significantly different from that predicted by the mean-field theory of Wang and Rice; notably, the liquid–vapor critical point predicted by the mean-field theory is too high. In contrast, the mean-field theory prediction of the density profile along the normal to the surface is rather good. When the surface coverage is small, the conformations of the chains are sensibly independent of density. However, as the surface coverage increases the interchain interactions compress the chain parallel to the surface and force extension of the chain along the normal to the surface. Even at infinite dilution significant portions of the chains lie in the planes above the plane in which the head group lies.