Diffusive hydrogen inter-cage migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates

Classical equilibrium molecular dynamics (MD) simulations have been performed to investigate
the diffusive properties of inter-cage hydrogen migration in both pure hydrogen and mixed
hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar from 200 K and up to 250-260 K. For mixed H2-
THF systems in which there is single H2 occupation of the small cage (labelled ‘1SC 1LC’), we
found that no H2 migration occurs. However, for more densely-filled H2-THF and pure- H2 systems,
in which there is more than single H2 occupation in the small cage, there is an onset of inter-cage H2
migration events from the small cages to neighbouring cavities at around 200 K. The mean square
displacements of the hydrogen molecules were fitted to a mathematical model consisting of an
anomalous term and a Fickian component, and non-linear regression fitting was conducted to
estimate long-time (inter-cage) diffusivities. An approximate Arrhenius temperature relationship for
the diffusion coefficient was examined and a rough estimation of the hydrogen hopping energy
barrier was calculated for each system.