Guo Guang-Jun, Zhang Yi-Gang, Zhao Ya-Juan, Refson Keith, Shan Gui-Hua
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China.
J Chem Phys. 2004 Jul 15;121(3):1542-7. doi: 10.1063/1.1763888.
Molecular dynamics simulations were performed to observe the evolution of cagelike water clusters immersed in bulk liquid water at 250 and 230 K. Totally, we considered four types of clusters--dodecahedral (5(12)) and tetrakaidecahedral (5(12)6(2)) cagelike water clusters filled with or without a methane molecule, respectively. The lifetimes of these clusters were calculated according to their Lindemann index (delta) using the criterion of delta> or =0.07. The lifetimes of the clusters at 230 K are longer than that at 250 K, and their ratios are the same as the ratio of structure relaxation times of bulk water at these temperatures. For both the filled and empty clusters, the lifetimes of 5(12)6(2) cagelike clusters are similar to that of 5(12) cagelike clusters. Although the methane molecules indeed make the filled cagelike water clusters live longer than the empty ones, the empty cagelike water clusters still have the chance of being long lived. These observations support the cluster nucleation hypothesis for the formation mechanisms of gas hydrates.
进行了分子动力学模拟，以观察在250K和230K下浸没在大量液态水中的笼状水团簇的演化。我们总共考虑了四种类型的团簇——分别填充或未填充甲烷分子的十二面体（5(12)）和十四面体（5(12)6(2)）笼状水团簇。根据它们的林德曼指数（δ），使用δ≥0.07的标准计算这些团簇的寿命。230K时团簇的寿命比250K时更长，它们的比值与这些温度下大量水的结构弛豫时间的比值相同。对于填充和空的团簇，5(12)6(2)笼状团簇的寿命与5(12)笼状团簇的寿命相似。尽管甲烷分子确实使填充的笼状水团簇比空的笼状水团簇寿命更长，但空的笼状水团簇仍有机会长时间存在。这些观察结果支持了气体水合物形成机制的团簇成核假说。
