Molecular dynamics simulation of high-pressure phases of gas hydrate solid solution

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The molecular dynamics (MD) method was applied to CH4-CO2 mixed hydrates with the structures sII (cubic) and filled ice (orthorhombic), in order to reproduce and predict the compositional dependence of crystallographic and thermodynamic properties.The following three patterns of sII mixed hydrate crystals were prepared for the simulation :1. The CH4 is completely partitioned into S-cage, and CO2 into L-cage.2. The CH4 and CO2 are evenly distributed to S- and L-cages (Disordered structure).3. The CO2 is completely partitioned into S-cage, and CH4 into L-cage.The system contains 192 gas molecules and 1088 H2O molecules. The MD calculation was carried out under constant temperature and pressure conditions (P=0.25 GPa, T=100,250,and 300 K). The characteristic results for sII mixed hydrate are as follows :1. The lattice parameter varies systematically with the CO2 concentration [=CO2/(CH4+CO2)] in S- and L-cages. The concentration of CO2 in S-cage largely affects the lattice parameter.2. Under low temperatures (100 and 250 K), there occurs a preferred orientation of CO2 molecules, which lowers the symmetry of the crystal from cubic to tetragonal.3. The structure with the enrichment of CO2 into L-cage is more stable than the disordered structure, where CH4 and CO2 are evenly distributed to both cages.The system with filled ice structure employed in this study contains 64 gas molecules and 128 H2O molecules. (Filled ice structure has only a kind of site that can contain gas molecules.) The MD calculation was carried out under constant temperature and pressure conditions (P=5 GPa, T=100 and 250 K).This filled ice structure is maintained only with the CO2 concentration less than 20%. In this range, the lattice parameters a, b, and c vary systematically with the CO2 concentration.