Molecular dynamics simulation of gas hydrate solid solution

采用分子动力学（MD）方法对CH_4-CO_2混合水合物（结构为I， Pm3n）进行了分析，得出并预测了其晶体学和热力学性质与组分的依赖关系。制备了以下三种类型的混合水合物晶体进行模拟：CH_4完全划分为s笼，CO_2完全划分为m笼。CH_4和CO_2均匀分布于S-笼和m -笼（无序结构）。CO_2完全划分为s笼，CH_4完全划分为m笼。该体系包含216个气体分子（s笼=54 + m笼= 162）和1242个h2o分子。本文采用的原子间势模型是一种经验模型，可以再现H_2O的介电常数、扩散系数和密度，CH_4和CO_2的振动谱、密度和结构。在一定压力和温度下，步进时间为0.4 fs，进行了MD计算。特征结果如下：1。在M-笼和s -笼中，随着CO_2浓度[=CO_2/(CH_4+CO_2)]的变化，晶格参数有系统的变化。s笼中CO_2浓度对晶格参数影响较大。组分对可压缩性的依赖性s笼中CO_2富集的晶体具有较高的可压缩性。水合物CO_2的体积模量比水合物CH_4的体积模量小10%左右。CO_2富集于m -笼的结构比CH_4和CO_2均匀分布于两个笼的无序结构更稳定。这与使用拉曼光谱测量笼中占用率的结果一致（Nakano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).