Molecular dynamics simulations of properties of a (0 0 1) methane clathrate hydrate surface

Abstract

The results of molecular dynamics simulations of a (0 0 1) methane clathrate hydrate surface interfaced with methane gas involving 2944 water and 512 methane molecules are reported for a temperature of 270 K. The water–water, water–methane and methane–methane interactions are evaluated using the flexible potential KKY. The program MXDORTO was used for the MD calculations and a newly developed code HBTOPOLOGY for the analysis of hydrogen bonds and the oxygen network. The oxygen and carbon surface z-density profiles present regular and periodic peaks that permit the decomposition of the system in slabs. On both surfaces, the external oxygen peak is depleted, indicating the formation of adlayers by water molecules on the surface. Various structural parameters for water molecules (orientational order and coordination) and oxygen atoms (mean-square-displacement, translational order, radial distribution function and ring distribution) are analysed in the different slabs. The distribution of the torsion angle between water dipole of neighbouring water molecules is introduced as a measure of the water molecules’ rearrangement on the surface. The analysed MD data indicate that rearrangements at the surface are mainly induced by adsorbed water molecules with a tendency to complement the open large cages. The analysis of jump frequencies (typically 3 ps) and distances (between 1.5 and 6 Å) indicates that the water molecules’ mobility should be observable in a time-of-flight quasi-elastic neutron scattering experiment.