In situ neutron diffraction studies of methane hydrate formation and decomposition

The hydration structure around methane hydrate during formation and decomposition has been determined using neutron diffraction with isotopic substitution. Measurements were made over a temperature range of 20 °C to 4 °C and at a constant pressure of 14.5 MPa. The first order difference from the neutron diffraction results suggests that there is an interesting two-stage process to methane hydrate formation that is not evident in the decomposition process. Empirical potential structure refinement (EPSR) of this new data, using the appropriate methane/water weightings for each time segment, confirms an earlier finding that methane hydrate formation increases the diameter of the hydration sphere by approximately 1 Å. In addition, the presence of solvent separated methane molecules is confirmed by the estimated C-C pair distribution function separation of 7 Å.