Molecular dynamics studies of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite

doi:10.1063/1.465067The effect of molecular steric properties on the melting of quasi‐two‐dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal‐plane surface of graphite. These molecules differ only in their length, being members of the n‐alkane series [CH3(CH2)n−2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule-substrate interactions. Nearest‐neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x‐ray scattering experiments. The melting of the low‐temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and the orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid HB phase transforms to a rectangular‐centered structure with a short coherence length in coexistence with a fluid phase. A significant result of the simulations is that they demonstrate the importance of molecular flexibility on the nature of the melting transition. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. The effect of molecular length on the qualitative nature of the melting process is discussed for both monolayers.This work was supported by The Danish Natural Science Research Council Grant No. M 11-7015, the U.S. NSF Grants No. DMR-8704938 and No. DMR-9011069,and the Pittsburgh Supercomputing Center Grant No. DMR-880008P