Microscopic Molecular Insights into Hydrate Formation and Growth in Pure and Saline Water Environments

The growth dynamics of natural gas hydrates in saline water has been studied using copious experiments and spectroscopic observations; however, the microscopic evidences to the structural and molecular transformations that they have provided are poorly understood. In this view, we perform extensive molecular dynamics simulations to gain physical insights into the formation and growth mechanism of naturally occurring gas hydrates with a wide variation in the amount of methane (1:5 to 1:18 methane/water ratio) in pure and salt (0-5 wt %) water environments at 50 MPa and 260 K. A couple of new findings analyzed from the number of cages and F_4φ order parameter are as follows: (a) 1:6 (methane/water ratio) is an optimum ratio for the rapid growth of a properly ordered hydrate in pure water at which the hydrate growth retards with increasing salt concentration, (b) there is an inconsequential difference between methane hydrate dynamics in pure water and 0.8 and 1.5 wt % salt water at a ratio of 1:12 (methane/water), and (c) lower methane (1:18) and salt (0.8 wt %) concentrations promote hydrate growth. Besides, this study observes the structural coexistence of S-I and S-II methane hydrates as the large 5¹²6⁴ cages appear along with the small 5¹² and large 5¹²6² cages, in which the low methane concentration favors the S-II structure.