|Title:||Water Vacancy Driven Diffusion in Clathrate Hydrates: Molecular Dynamics Simulation Study||Authors:||Lo H.
|Issue Date:||2017||Journal Volume:||121||Journal Issue:||15||Start page/Pages:||8280-8289||Source:||Journal of Physical Chemistry C||Abstract:||
Water vacancy is a defect in clathrate hydrates which facilitates gas diffusion by lowering the energy barrier for the cage hopping transfer of guest molecules. By performing molecular dynamics (MD) simulation on single phase sI CH4 and CO2 hydrates, we explore the dynamics of guest hopping and the concurrent evolution of a hydrogen bond network on the hopping sites. The inclusion of the water vacancy allows us to observe microscopic mechanisms of gas hopping in stable (i.e., away from dissociation temperature) hydrates within 100 ns simulations. Diffusion coefficients of guest gases are estimated from the obtained hopping rate, which have qualitative agreement with literature experimental data and simulation calculations (estimated CO2 diffusivities are 3.6 ¡Ñ 10-16 m2/s at 273 K and 1.1 ¡Ñ 10-15 m2/s at 288 K). The single water vacancy defect causes on average four damaged cages and propagates in the system with a diffusivity of ca. 10-12 m2/s. When the defect locates between an occupied and an unoccupied cage, gas hopping to the unoccupied cage may occur. By increasing the concentration of the defect, we observe a unique self-assembled behavior of the damaged cages, which were originally separated, clustering into fewer but larger damaged domains. These domains significantly increase the hopping frequency of guest molecules, and the diffusivity of gas within the domains equals ca. 10-12 m2/s calculated by the Einstein equation. ? 2017 American Chemical Society.
|Appears in Collections:||化學工程學系|
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