Effects of chain branching on the structure of interfacial films of decane isomers

Abstract

This work is a molecular dynamics simulation study of the influence of chain branching on the structures of molecular films physically adsorbed on solid surfaces. The model systems considered are free-standing films of three C10H22 isomers: n-decane, i-decane (2-methylnonane), and t-butyl-hexane (2,2-methyloctane), on a Pt(111) surface. Proper potential models for the t-butyl group were developed and verified. All of these films exhibit layering and in-layer ordering near the surface and we have quantified these features. In general, layered molecules tend to lie with their molecular axes and backbone planes parallel to the surface. In-layer ordering is observed for molecules in layers near the surface. Neighboring molecules within ordered layers tend to align with their long molecular axes parallel to each other. n-Decane, a symmetric chain molecule, exhibits the strongest layering and in-layer ordering. The structures of i-decane films are very similar to those of n-decane films and show only slightly less order. t-Butyl-hexane films have a novel pillared layered structure, in which a few randomly distributed molecules orient themselves with the t-butyl end near the surface and the alkyl tail perpendicular to the surface. These molecules are surrounded by parallel, layered molecules. We discuss the implications of our findings for solvation forces in confined thin films of these molecules.