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

Abstract

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 experimentally. 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; here, not unexpectedly, double occupancies occurred more favorably in large cages and single occupancies dominated in small cages.

Analysing the stability of He-filled hydrates: how many He atoms fit in the sII crystal?

“…47 Additionally, a similar behaviour has been observed for other small guests, such as hydrogen. [74][75][76][77] Therefore, in order to shed more light on this matter, three different He-filled sII structures will be investigated in this work: He (1/1) @sII, He (1/4) @sII and He (2/4) @sII. The empty sII crystal will also be considered in order to check the influence of the He guests on the lattice.…”

A comprehensive review on molecular dynamics simulation studies of phenomena and characteristics associated with clathrate hydrates

“…Based on MD analysis of H2 and D2 molecular scape from the hydrate phase using the same force-field parameters at 150 to 195 K, it was also revealed that the stronger collisions between the cage and guest molecules may increase the likelihood of cage distortion which results in the subsequent escape of D2 molecules. Also, the leakage rates of H2 were observed to be lower than those for D2 [218]. In addition, the in-slab translational order parameters of the CH4 hydrate surface in contact with the gas phase determined that the molecules of water in the adlayers and slab surfaces can be periodically arranged while the internal slabs can remain completely rigid [97].…”