Molecular dynamics simulations have been used to study the structure and dynamics of monolayers of long-chain molecules on a metallic substrate. The system consisted of 90 molecules held at a fixed surface density and periodically replicated in the plane of the surface. Two models have been studied as alternative representations of the admolecule–surface interactions in layers formed by the self-assembly of alkyl thiol [SH(CH2)15CH3] molecules onto a gold substrate. The principal difference between the two models is that in one, the S–C bond is required to lie nearly parallel to the substrate surface. After lengthy equilibration at room temperature, both models yield monolayers in which the chains are aligned and tilted with respect to the surface normal. Although the tilt angle is different for the two models, the thickness of the monolayers is the same and the influence of the ‘‘modified-headgroup’’ on the detailed structure of the film is confined to the region closest to the metal surface. There are striking differences in the chain rotational dynamics of the two models which are likely amenable to experimental verification.

No abstract

No abstract

Molecular dynamics calculations have been used to study the effects of temperature on a dense monolayer of hydrocarbon molecules. The simulation system consisted of 90 flexible chains with headgroups and had periodic boundary conditions in the plane of the surface. The interaction potentials were chosen to model a monolayer of chemisorbed alkyl thiol molecules [S(CH2)15CH3] on a Au(111) surface; molecules that self-assemble from solution to form a triangular lattice at a fixed surface density of 21.4 Å2 per chain. Simulations at different temperatures reveal distinct phases with different kinds of disorder. Nonequilibrium molecular dynamics techniques have been used to investigate the transition from the high-temperature state, in which the molecular planes undergo reorientational motion, to a low-temperature, orientationally ordered state. Possible correlations between the rotational phase transition and the appearance of conformational defects are also explored.

A b initio molecular orbital pair potentials for the interaction of Fe2+ and Fe3+ ions with H2O are reported. Molecular dynamics calculations of the static structure of the solvation shell of Fe2+ and Fe3+ in water using the ab initio pair potentials gives physically incorrect results, i.e., the coordination numbers are eight instead of six as observed experimentally. This problem has also been encountered by other workers for divalent transition metal ions in water. By computing three-body energies from the interaction of two water molecules with the cations, we show that the origin of the problem is most likely in the assumption of the additivity of the pair potentials, i.e., neglect of many-body forces. Empirical potentials are reported which take approximate account of the three-body forces and give coordination numbers of six for both Fe2+ and Fe3+ in water.