Zhang Zhengcai, Guo Guang-Jun
Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China.
Phys Chem Chem Phys. 2017 Jul 26;19(29):19496-19505. doi: 10.1039/c7cp03649c.
Although ice powders are widely used in gas hydrate formation experiments, the effects of ice on hydrate nucleation and what happens in the quasi-liquid layer of ice are still not well understood. Here, we used high-precision constant energy molecular dynamics simulations to study methane hydrate nucleation from vapor-liquid mixtures exposed to the basal, prismatic, and secondary prismatic planes of hexagonal ice (ice Ih). Although no significant difference is observed in hydrate nucleation processes for these different crystal planes, it is found, more interestingly, that methane hydrate can nucleate either on the ice surface heterogeneously or in the bulk solution phase homogeneously. Several factors are mentioned to be able to promote the heterogeneous nucleation of hydrates, including the adsorption of methane molecules at the solid-liquid interface, hydrogen bonding between hydrate cages and the ice structure, the stronger ability of ice to transfer heat than that of the aqueous solution, and the higher occurrence probability of hydrate cages in the vicinity of the ice surface than in the bulk solution. Meanwhile, however, the other factors including the hydrophilicity of ice and the ice lattice mismatch with clathrate hydrates can inhibit heterogeneous nucleation on the ice surface and virtually promote homogeneous nucleation in the bulk solution. Certainly, the efficiency of ice as a promoter and as an inhibitor for heterogeneous nucleation is different. We estimate that the former is larger than the latter under the working conditions. Additionally, utilizing the benefit of ice to absorb heat, the NVE simulation of hydrate formation with ice can mimic the phenomenon of ice shrinking during the heterogeneous nucleation of hydrates and lower the overly large temperature increase during homogeneous nucleation. These results are helpful in understanding the nucleation mechanism of methane hydrate in the presence of ice.
尽管冰粉广泛应用于气体水合物形成实验中，但冰对水合物成核的影响以及冰的准液层中发生的情况仍未得到很好的理解。在此，我们使用高精度等能分子动力学模拟，研究了暴露于六方冰（冰Ih）基面、棱柱面和次棱柱面的气液混合物中甲烷水合物的成核过程。尽管在这些不同晶面的水合物成核过程中未观察到显著差异，但更有趣的是发现甲烷水合物既可以在冰表面异相成核，也可以在本体溶液相中均相成核。文中提到了几个能够促进水合物异相成核的因素，包括甲烷分子在固液界面的吸附、水合物笼与冰结构之间的氢键、冰比水溶液更强的传热能力以及冰表面附近水合物笼出现的概率高于本体溶液。然而，与此同时，包括冰的亲水性和冰晶格与笼形水合物的不匹配等其他因素会抑制冰表面的异相成核，并实际上促进本体溶液中的均相成核。当然，冰作为异相成核促进剂和抑制剂的效率是不同的。我们估计在工作条件下前者大于后者。此外，利用冰吸收热量的特性，用冰进行水合物形成的NVE模拟可以模拟水合物异相成核过程中冰收缩的现象，并降低均相成核过程中过大的温度升高。这些结果有助于理解冰存在下甲烷水合物的成核机理。
