Effect of Methane Adsorption on the Lifetime of a Dodecahedral Water Cluster Immersed in Liquid Water:  A Molecular Dynamics Study on the Hydrate Nucleation Mechanisms

By performing 400 molecular dynamics simulations, we observe that the rigid dodecahedral water cluster (DWC) encaging a methane molecule has an affinity for other methane molecules diffusing freely in liquid water and can adsorb them by chance. Utilizing the affinity, we successfully prepare seven series (200 runs in each series) of DWCs adsorbing 0−12 methane molecules, and study how the adsorbed methane molecules affect the DWC lifetime. The results show that the adsorbed methane molecules can prolong the lifetime of DWC. The structures and dynamics of the DWC and its surroundings are also analyzed. We find that the adsorbed methane molecules slow down the water molecules in both the DWC and its surroundings, reduce the number of H-bonds that tend to break the DWC, and change the H-bond networks of the DWC surroundings from the liquid water structure toward the cagelike structure. On the basis of these observations, a methane aggregation mechanism caused by the DWC adsorbing dissolved methane molecules is newly proposed, which may help in understanding the nucleation process of methane hydrate.