Molecular dynamics simulation of gas hydrate solid solution

将分子动力学方法应用于CH_4-CO_2混合水合物（结构I，Pm 3 n），以便产生和预测晶体学和热力学性质的组成依赖性。1.CH_4完全分配到S笼中，CO_2完全分配到M笼中，2.CH_4和CO_2均匀分配到S笼和M笼中（无序结构）.3.CO_2完全被分配到S笼中，该体系含有216个气体分子（=54个S笼+162个M笼）和1242个H_2O分子。本文采用的原子间相互作用势模型是一个经验模型，它能再现H_2O的介电常数、扩散系数和密度，以及CH_4和CO_2的振动谱、密度和结构。在一定的温度和压力下进行了分子动力学计算，计算步长为0.4fs，得到了以下特征性结果：1.晶格参数的组成依赖性M笼和S笼中的晶格参数随CO_2浓度[=CO_2/（CH_4+CO_2）]系统地变化。C笼中CO_2的浓度对晶格参数有很大的影响。2.压缩率的组分依赖性C笼中CO_2浓度高的晶体具有较高的压缩率。CO_2水合物的体积弹性模量比CH_4水合物的体积弹性模量小10%左右。3.摩尔模量的组成依赖性CO_2富集在M笼中的结构比CH_4和CO_2均匀分布在两个笼中的无序结构更稳定。这与通过使用拉曼光谱测量笼占有率的结果一致（中野和Ohgaki，2000）。

The molecular dynamics (MD) method was applied to CH_4-CO_2 mixed hydrate (structure I, Pm3n) in order to produce and predict the compositional dependence of crystallographic and thermodynamic properties.The following three types of mixed hydrate crystals were prepared for the simulation:1.The CH_4 is completely partitioned into S-cage, and CO_2 into M-cage.2.The CH_4 and CO_2 are evenly distributed to S- and M-cages (Disordered structure).3.The CO_2 is completely partitioned into S-cage, and CH_4 into M-cage.The system contains 216 gas molecules (=54 in S-cage + 162 in M-cage) and 1242 H_2O molecules. The interatomic potential model employed in this study is an empirical one, which can reproduce the dielectric constant, diffusion coefficient, and density for H_2O, the vibration spectrum, density, and structure for CH_4 and CO_2. The MD calculation was carried out under some pressures and temperature with the step time of 0.4 fs.The characteristic results are as follows:1.The compositional dependence of lattice parameterThe lattice parameter varies systematically with the CO_2 concentration [=CO_2/(CH_4+CO_2)] in M- and S-cages. The concentration of CO_2 in S-cage largely affects the lattice parameter.2.The compositional dependence of compressibilityThe crystals with the enrichment of CO_2 in S-cage have high compressibility. The bulk modulus of CO_2 hydrate is about 10% smaller than that of CH_4 hydrate.3.The compositional dependence of molar enthalpyThe structure with the enrichment of CO_2 into M-cage is more stable than the disordered structure, where CH_4 and CO_2 are evenly distributed to both cages. This is consistent with the result of cage occupancy measurements by use of Raman spectroscopy (Nakano and Ohgaki, 2000).