No abstract

Neutron diffraction with HD isotope substitution has been used to study the formation and decomposition of the methane clathrate hydrate. Using this atomistic technique coupled with simultaneous gas consumption measurements, we have successfully tracked the formation of the sI methane hydrate from a water/gas mixture and then the subsequent decomposition of the hydrate from initiation to completion. These studies demonstrate that the application of neutron diffraction with simultaneous gas consumption measurements provides a powerful method for studying the clathrate hydrate crystal growth and decomposition. We have also used neutron diffraction to examine the water structure before the hydrate growth and after the hydrate decomposition. From the neutron-scattering curves and the empirical potential structure refinement analysis of the data, we find that there is no significant difference between the structure of water before the hydrate formation and the structure of water after the hydrate decomposition. Nor is there any significant change to the methane hydration shell. These results are discussed in the context of widely held views on the existence of memory effects after the hydrate decomposition.

The difference methods of neutron diffraction and isotopic substitution (NDIS) were applied to the total scattering patterns of H/D substituted solutions containing three types of phosphate anion (H 2 PO 4 À , HPO 4 2À , PO 4 3À). The data were used to determine the pairwise structure associated with the hydrogen and oxygen atoms of the solution. The results are consistent with hydrogen bonds between the H 2 PO 4 À , HPO 4 2À and PO 4 3À anions and water molecules being respectively stronger than, comparable to and weaker than those between the hydrogen bonds of water molecules. It is also deduced that the coordination number for the PO 4 3À ion is 15 AE 3, and a similar value is proposed for the other two anions.