The crystalline structures of the even alkanes hexane, octane, decane, dodecane and tetradecane monolayers adsorbed on graphite at submonolayer coverages and from the liquid

Abstract

Neutron and X-ray diffraction have been used to structurally characterise the crystalline monolayer structures of hexane, octane, decane, dodecane and tetradecane adsorbed on graphite at sub-monolayer coverages and when coexisting with liquid alkane. The structures of all the molecules investigated at both coverages and low temperatures are isomorphous with rectangular unit cells of plane group pgg containing two molecules per cell. In both high- and low-coverage structures the molecules have their extended axes parallel to the surface. The plane of the carbon skeleton is found to be parallel to the graphite surface. The monolayers at sub-monolayer coverages are interpreted as uniaxially commensurate while those monolayers coexisting with the liquid are fully commensurate. Dodecane and tetradecane are exceptional: dodecane forms additional phases at high temperatures just prior to melting, at both low and high coverages. In these structures the molecules in the unit cell are parallel to each other with plane group cmm. Tetradecane only forms a single phase at high coverages in which the molecules appear to be parallel and upright, similar to the dodecane high temperature, high coverage phase.