Guang-Jun Guo - Why can water cages adsorb aqueous methane? A potential of mean force calculation on hydrate nucleation mechanisms

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      Publication Details (including relevant citation   information):

      Guo, G., Li, M., Zhang, Y., and Wu, C., 2009, Physical Chemistry   Chemical Physics, Vol.11, pp.10427-10437.


      By performing constrained molecular dynamics simulations in the   methane–water system, we successfully calculated the potential of   mean force (PMF) between a dodecahedral water cage (DWC) and   dissolved methane for the first time. As a function of the   distance between DWC and methane, this is characterized by a deep   well at ~6.2 Å and a shallow well at ~10.2 Å, separated by a   potential barrier at ~8.8 Å. We investigated how the guest   molecule, cage rigidity and the cage orientation affected the   PMF. The most important finding is that the DWC itself strongly   adsorbs methane and the adsorption interaction is independent of   the guests. Moreover, the activation energy of the DWC adsorbing   methane is comparable to that of hydrogen bonds, despite   differing by a factor of ~10% when considering different   water–methane interaction potentials. We explain that the   cage–methane adsorption interaction is a special case of the   hydrophobic interaction between methane molecules. The strong net   attraction in the DWC shell with radii between 6.2 and 8.8 Å may   act as the inherent driving force that controls hydrate   formation. A cage adsorption hypothesis for hydrate nucleation is   thus proposed and discussed.

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