Electric-Field Control of Neon Uptake and Release to and from Clathrate Hydrates

Abstract

The thermodynamic and kinetic properties of neon uptake and release to and from a sII clathrate hydrate were evaluated using molecular dynamics simulation in the absence and presence of an applied static electric field. In the case of neon "leakage", neon clathrate in contact with a vacuum was simulated at temperatures ranging from 200 to 225 K for 0.5 μs, with progressive neon-emptying of cages monitored. Activation energies for release and uptake were 16.4 and 14.9 kJ/mol, respectivelyconsistent with experimentally determined values; for neon release in a 0.7 V/nm electric field, the value declines to 6.5 kJ/mol, suggesting its use as a control agent in facilitating gas release from alreadyloaded clathrates. For neon uptake into an initially empty hydrate, loading free energies for 5 12 and 5 12 6 4 cages, computed via thermodynamic integration, was lower than that previously reported for methane uptake into sI hydrates.

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

“…This provided a quantitative check on lattice integrity, as well as allowing for a quantitative study of guest numbers’ depletion in the hydrate phase (in tandem with vacuum-phase H 2 /D 2 -molecule counting and visualization). Above around 180 K (i.e., in the 185–195 K region), there was the onset of bulk-lattice dissociation for the variously occupied hydrate-lattice systems (i.e., thermodynamic melting), albeit even after a good fraction of a microsecond, so simulations with these moderate thermodynamic thermal driving forces were not analyzed: as with ref ( 34 )., the purpose of the present study is to investigate nonequilibrium cage-hop-mediated diffusional escape of H 2 /D 2 guests under conditions in which the “leaky” lattice itself is thermodynamically (meta)stable and certainly kinetically so, over simulation timescales of at least a microsecond. As with ref ( 34 ).…”

“…Above around 180 K (i.e., in the 185–195 K region), there was the onset of bulk-lattice dissociation for the variously occupied hydrate-lattice systems (i.e., thermodynamic melting), albeit even after a good fraction of a microsecond, so simulations with these moderate thermodynamic thermal driving forces were not analyzed: as with ref ( 34 )., the purpose of the present study is to investigate nonequilibrium cage-hop-mediated diffusional escape of H 2 /D 2 guests under conditions in which the “leaky” lattice itself is thermodynamically (meta)stable and certainly kinetically so, over simulation timescales of at least a microsecond. As with ref ( 34 ). for the case of neon’s leaky escape from sII clathrate, this allows for plateaux in guest-release numbers to be realized (i.e., essentially attainment of guest chemical-potential equilibrium in both hydrate and vacuum phases).…”

“…In this approach, the Hamiltonian H is modified as a function also of a coupling parameter, λ, i.e., H = H (p; q; λ), in such a way that λ = 0 describes system A (decoupled) and λ = 1 describes system B (coupled). 34 …”

“…In the present study, given this broad and well-justified interest in hydrogen hydrates as a hydrogen-storage material, we are motivated by unresolved, open questions relating to both H 2 and D 2 intercage diffusivity and release of the H 2 and D 2 to vacuum, given our observation of “leaky” neon sII hydrates. 34 We wish to understand if such leaks extend to H 2 and D 2 . In particular, with a guest-loaded hydrate and therefore a chemical-potential driving force for guest mass transfer (via Fick’s Law), it is fascinating to scrutinize the phenomena underpinning transfer of guest molecules passing from one cage to another as a dramatic case study of nonequilibrium diffusion.…”

“…In addition, to characterize driving force, the free-energy profile was also calculated using thermodynamic integration for all occupancies. 34 …”