The Quasi-Liquid Layer of Ice under Conditions of Methane Clathrate Formation

Abstract

A premelted layer of water wets the surface of ice at temperatures below the melting temperature. Experiments suggest that this quasi-liquid layer may play an important role in the nucleation of clathrate hydrates from ice. Nevertheless, the structure of the quasi-liquid layer of ice in the presence of methane or other clathrate-forming gases has not yet been elucidated. In this work, we perform large-scale molecular dynamic simulations with a coarse-grained molecular model to investigate the properties of the quasi-liquid layer of ice in the presence of methane gas under pressure. We characterize the structure and thickness of the ice/methane and ice/vacuum interfaces, and the solubility of methane in the premelted layer as a function of temperature. We find that the width of the quasi-liquid layer fluctuates between 5 and 45 Å in the presence of a methane-like solute at temperatures within 1 K of the melting point. The width of the quasi-liquid layer of ice at temperatures lower than 270 K is less than the diameter of a water dodecahedron, the smallest cage that constitutes the clathrates. The simulations indicate that, when the premelting layer is wider than 10 Å, the structure of water and solubility of methane in the center of the quasi-liquid layer are the same as in bulk liquid water at the same temperature. These results are relevant for understanding the mechanism of formation of methane hydrate clathrates from ice.