Molecular Dynamics of Langmuir-Blodgett Films: II. Bilayers

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Abstract
We have simulated Langmuir Blodgett bilayers of stearic acid at 25°C for two head-group areas using the molecular dynamics method. At Am = 21.2 Å2 the molecules are tilted at 18° to the surface normal with a next-nearest-neighbour azimuthal orientation. The head-group dipoles are parallel to the plane of the surface and antiparallel 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 head-groups. The head-groups exhibit a strong crystalline structure in the plane of the bilayer at both dessities. Increasing the density to 20.6 Å2 does not significantly alter the tilt angle, this contrast to the energy minimisation results and the molecular dynamics results for the monolayer at the same temperature and head-group area. It produces a distortion of the translational ordering of the molecular centers from the initial triangular arrangement towards a rectangular structure which is evocative of the crystal B structure.

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