Neutron diffraction from benzene adsorbed on graphite

Abstract

Neutron diffraction has been used to determine the structural properties of benzene adsorbed on graphitized carbon black at coverages of 0.4 to 1.2 monolayers and temperatures from 100 to 200 K. Benzene adsorbs on the basal planes of graphite to form a two-dimensional monolayer in which all the molecules lie flat, or very nearly so, with a molecular area of 37 Ų. At low coverages and low temperatures the molecules aggregate in two-dimensional clusters rather than spreading over the surface to form a lattice gas. Above a monolayer and at 100 K, any excess benzene in the system nucleates to form crystallites of normal orthorhombic benzene without disturbing the monolayer already present. There is therefore no structural phase change in the two-dimensional monolayer at higher coverages. At 200 K the benzene forms a two-dimensional fluid at all the coverages studied. At low coverages the molecules in the fluid remain flat on the surface but at higher coverages they have greater orientational freedom.