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

摘要

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