Simulation of a monolayer of alkyl thiol chains

Abstract

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.