Kainai Dilare, Bai Dongsheng
Department of Chemistry, College of Chemistry and Materials Engineering/Key Laboratory of Cosmetic, China National Light Industry, Beijing Technology and Business University, Beijing 100048, P. R. China.
J Phys Chem B. 2022 Jan 20;126(2):492-502. doi: 10.1021/acs.jpcb.1c07582. Epub 2022 Jan 5.
Gas hydrates usually contain a certain number of empty cages that will both affect the hydrate stability and reduce the gas storage capacity. In this work, by MD simulations, we found that the hydrate stability is related to the cage occupancy, the empty cage types, and especially the distribution of empty cages. With the decrease of overall occupancy, the stability of hydrate becomes worse. Under the same overall occupancy, the more concentrated the empty cages are, the more unstable the hydrate is and hence the faster it decomposes. The methane molecules may migrate between distorted cages during the decomposition, resulting in a temporary increase in the stability of hydrate. Hydrates with different empty cage distributions show different decomposition mechanisms: when empty cages are concentrated, the melting rate is fast first due to the rapid decomposition of empty cages, but the remaining filled cages will reduce the melting rate; when empty cages are separated on the contrary, the early melting is slow because of the high local occupancy, and the following melting will be accelerated because of the high melting surface area. It indicates that the empty cage distribution plays a controlling role in hydrate decomposition kinetics at different stages.
气体水合物通常含有一定数量的空笼，这既会影响水合物的稳定性，又会降低气体储存容量。在这项工作中，通过分子动力学（MD）模拟，我们发现水合物的稳定性与笼占有率、空笼类型，尤其是空笼的分布有关。随着总体占有率的降低，水合物的稳定性变得更差。在相同的总体占有率下，空笼分布越集中，水合物越不稳定，分解速度也就越快。在分解过程中，甲烷分子可能在变形笼之间迁移，导致水合物稳定性暂时增加。具有不同空笼分布的水合物表现出不同的分解机制：当空笼集中时，由于空笼的快速分解，初期熔化速度较快，但剩余的填充笼会降低熔化速度；相反，当空笼分散时，由于局部占有率高，初期熔化较慢，而随后由于熔化表面积大，熔化速度会加快。这表明空笼分布在水合物不同阶段的分解动力学中起控制作用。
