Water ordering around methane during hydrate formation

The structure of water around methane during hydrate crystallization from a queous solutions of methane is studied using neutron diffraction with isoto pic substitution over the temperature range 18 degrees C to 4 degrees C, an d at two pressures, 14.5 and 3.4 MPa. The carbon-oxygen pair correlation fu nctions, derived from empirical potential structure refinement of the data, indicate that the hydration sphere around methane in the liquid changes dr amatically only once hydrate has formed, with the water shell around methan e being about 1 Angstrom larger in diameter in the crystal than in the liqu id. The methane coordination number in the liquid is around 16 +/- 1 water molecules during hydrate formation, which is significantly smaller than the value of 21 +/- 1 water molecules found for the case when hydrate is fully formed. Once hydrate starts to form, the hydration shell around methane be comes marginally less ordered compared to that in the solution above the hy drate formation temperature. This suggests that the hydration cage around m ethane in the liquid may be different from that when hydrate is forming and from that found in the hydrate crystal structure. Methane-methane radial d istribution functions show that methane molecules can adopt a range of sepa rations during hydrate formation, corresponding to the more distorted natur e of the methane-water correlations. There is noticeable ordering of the me thane molecules with a monolayer of water molecules between them once hydra te has formed. The dipole moments of the hydrating water molecules lie most ly tangential to the methane-water axis, both before, during, and after hyd rate formation. (C) 2000 American Institute of Physics. [S0021-9606(00)7013 5-8].