Lifetimes of cagelike water clusters immersed in bulk liquid water: A molecular dynamics study on gas hydrate nucleation mechanisms

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 deltagreater than or equal to0.07. 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. (C) 2004 American Institute of Physics.