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

采用分子动力学（MD）方法对sII（立方）和填充冰（正交）两种结构的CH4-CO2混合水合物进行了研究，以重现和预测其晶体学和热力学性质对组分的依赖关系。制备了三种类型的sII混合水合物晶体进行模拟：CH4完全划分到s笼中，CO2完全划分到l笼中。CH4和CO2均匀分布于S-笼和l -笼（无序结构）。CO2完全分成s笼，CH4完全分成l笼。该系统包含192个气体分子和1088个水分子。MD计算在恒温常压条件下进行（P=0.25 GPa， T=100,250和300 K）。sII混合水合物的特征结果如下：在S-笼和l -笼中，晶格参数随CO2浓度[=CO2/(CH4+CO2)]有系统的变化。s笼中CO2浓度对晶格参数影响较大。在低温下（100和250 K）， CO2分子发生优先取向，这降低了晶体的对称性，从立方到四边形。l -笼中富集CO2的结构比无序结构更稳定，无序结构中CH4和CO2均匀分布在两个笼中。本研究采用的填充冰结构体系包含64个气体分子和128个H2O分子。（填充冰结构只有一种部位可以容纳气体分子。）MD计算是在恒温常压条件下（P=5 GPa， T=100和250 K）进行的。只有在二氧化碳浓度低于20%的情况下，才能维持这种填充冰结构。在此范围内，晶格参数a、b和c随着CO2浓度的变化而系统地变化。

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.