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Molecular dynamics calculations have been used to investigate collective molecular tilt as a function of area per chain at room temperature in substrate-supported monolayers of long-chain molecules. At the highest densities the chains align approximately normal to the substrate and show a continuous increase in the collective tilt as the molecular area is increased, a finding in agreement with recent experimental results on Langmuir monolayers.1•2 An intermolecular potential which represents the methylene group as a single interaction site ("united atoms") is compared with a potential in which all atoms are included explicitly. Commonly used parameters for the united atom potential underestimate the effective diameter of the chains.

Molecular-dynamics calculations have been carried out for a system of long-chain molecules supported on a physisorbing planar substrate and constrained to a surface density characteristic of that observed in lipid and fatty acid monolayers at the air-water interface. Results are presented for the density distribution normal to the surface, the structure factor parallel to the surface, and the number of chain conformational defects as the surface area per chain is increased at room temperature. The driving force for the observed structural and conformational changes appears to be related to the tendency for the alkyl-chain methylene groups to close-pack.

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Molecular dynamics simulations have been used to investigate diffusion in two commonly used industrial solvents, toluene and tetrahydrofuran. Several different models for the solvents are compared (flexible vs. rigid, all-atom vs. united atom), and it is found that united atom and all-atom models of the solvents produce very different diffusion coefficients at the experimental density. This disagreement can be explained by the pressure dependence of the diffusion coefficient, which is found to vary in accord with the Chapman-Enskog result for hard spheres. It is recommended that force fields be parametrized carefully to produce reasonable pressures at the experimental densities, or that simulations be carried out at constant pressure, if they are to be used for the purposes of calculating transport coefficients.