Structure H clathrate unit cell coordinates and simulation of the structure H crystal interface with water

Abstract

The Cartesian coordinates for the structure H clathrate unit cell have been determined to within 5% error. The unit cell, consisting of 34 water molecules and 6 hydrocarbon molecules, was replicated to create a system amenable to molecular dynamics simulation. Radial distribution functions were determined for this crystal. The method of molecular dynamics was employed to model the interface between the crystal and liquid water. It was found that the interfacial behavior spanned a region of 15 Å on both sides of the interface. Both orientational and translational motion in the liquid phase became more vibrational as the interface was approached, while the temperature profile remained constant within about 15° across the interface. In addition, there was a small tendency for the molecules in the liquid phase of the interfacial region to align themselves in an ordered, solidlike configuration. The solid region within about 10 Å of the interface demonstrated persistent steady-state disorder compared with the solid bulk phase.