Diffusion Analysis and Structure Analysis during <i>CO</i>₂ Hydrate and <i>CH</i>₄ Hydrate Dissociation and Formation Conditions

Gas hydrates are ice-like water structures that contain gas molecules inside their cavities. Gas hydrates with methane and carbon dioxide as guest molecules form structure I hydrates containing 6 large cages and 2 small cages per unit cell. This study uses molecular dynamics simulations to assess the diffusivity of methane and carbon dioxide released from dissociated hydrates and investigates the behavior of these dissociated molecules, methane, water, and carbon dioxide, under hydrate formation conditions by analyzing the radial distribution function (RDF) and mean square displacement of the gas–water systems. The dissociation process simulation uses an ideal 2 × 2 × 2 supercell with a stepwise temperature rise as described in our previous work. Both methane and carbon dioxide hydrates at 100% occupancy and a 75% occupancy of carbon dioxide are tested. The result shows that the diffusion coefficient of the 100% occupied carbon dioxide hydrate is larger than that of the 100% occupied methane hydrate, but the coefficient for the 75% occupancy CO2 hydrate was significantly lower than that for the 100% occupancy methane hydrate. The hydrate formation simulations are performed over predissociated 4 × 4 × 4 methane hydrate and carbon dioxide hydrate systems, under formation conditions for 5 and 10 ns. The analysis includes the transport and water coordination properties determined using the mean square displacement of O2, CH4, and CO2 and the RDF of O2 atoms. The results show that in all cases, the TIP4P oxygen–oxygen RDF exhibited pronounced valleys, and this behavior became more prominent at 10 ns. The RDF when using the TIP4P/ICE force field remained relatively unchanged throughout the simulations, which could be caused by its larger depth of the energy well ϵ parameter as compared to that for the TIP4P/ICE force field.