MOLECULAR-DYNAMICS OF LANGMUIR-BLODGETT-FILMS .2. BILAYERS

We have simulated Langmuir Blodgett bilayers of stearic acid at 25-deg rees-C for two head-group areas using the molecular dynamics method. A t A(m) = 21.2 angstrom2 the molecules are tilted at 18-degrees to the surface normal with a next-nearest-neighbour azimuthal orientation. Th e head-group dipoles are parallel to the plane of the surface and anti parallel to each other. The precise ordering of the dipoles depends on the density of the bilayer. There is a strong correlation between the translational and orientational ordering of the molecules in the two layers due to strong inter-layer electrostatic coupling between the he ad-groups. The head-groups exhibit a strong crystalline structure in t he plane of the bilayer at both densities. Increasing the density to 2 0.6 angstrom2 does not significantly alter the tilt angle, this contra st to the energy minimisation results and the molecular dynamics resul ts for the monolayer at the same temperature and head-group area. It p roduces a distortion of the translational ordering of the molecular ce ntres from the initial triangular arrangement towards a rectangular st ructure which is evocative of the crystal B structure.