Hydrogen and Deuterium Molecular Escape from Clathrate Hydrates: "Leaky" Microsecond-Molecular-Dynamics Predictions

It is predicted herewith that the leakage of both hydrogen (H-2) and deuterium (D-2) from sII clathrate hydrates, borne of guest chemical-potential equalization driving enhanced nonequilibrium intercage hopping, should be observable exper-imentally. To this end, we have designed simulations to realize and study this process by microsecond molecular dynamics within the temperature range of 150-180 K-for which the hydrate lattice was found to be stable. In this pursuit, we considered initial large-cage (5(12)6(4)) guest occupancies of 1-4, with single occupation of 5(12) cavities. Examining transient, nonequilibrium intercage hopping, we present a lattice-escape activation energy for the four nominal large-cage occupancies (1-4), by fitting to the hydrate-leakage rate. The intercage hopping of H-2 and D-2 was studied using Markov-chain models and expressed at different temperatures and large-cage occupancies. The free energy of guest "binding" in the large and small cages was also computed for all of the occupancies. Toward equilibrium, following the majority of H-2/D-2 escape via leakage, the percentage of occupancies was calculated for both H-2 and D-2 for all of the systems for all initial nominal large-cage occupancies; her; not unexpectedly, double occupancies occurred more favorably in large cages and single occupancies dominated in small cages.