| Résumé | In this work, the effect of cyclopentane (CP) clathrate hydrate on the anomalous preservation of tetrahydrofuran (THF) hydrate under conditions outside its stability region is studied by using molecular dynamics simulations. The decompositions of pure structure II THF and CP clathrate hydrate, and also THF hydrate coated by CP hydrate, all with outer (001) surfaces exposed to vacuum, were simulated at different temperatures and characterized by the potential energy, the F3 order parameter, and visual inspection of snapshots of the hydrate system at different times. The upper bounding melting points of THF and the CP hydrate with the employed force fields were predicted to be 270 and 290 K, respectively, which were close to the experimental values of 277.5 and 281 K. To study the origins of anomalous preservation and superheating effects in hydrates, we placed layers of the higher-decomposition point CP hydrate as a coating on bulk THF hydrate to study the possible superheating of the THF hydrate. Whereas the pure THF hydrate melted at 270 K after a simulation time of about 50 ns, with the CP hydrate layer coating, the THF hydrate in the simulations did not dissociate at 290 K, corresponding to a superheating temperature of 20 K, up to a simulation time of 120 ns. Upon coating with the CP hydrate, the decomposition of the THF hydrate is transformed from a heterogeneous mechanism at the hydrate–vacuum or hydrate–water interface to a homogeneous mechanism which leads to superheating of the THF hydrate phase. The dissociation of the THF and the CP hydrate layers occurs in a stepwise fashion perpendicular to the hydrate interface from the outer to inner layers, similar to the dissociation of the structure I methane hydrate previously studied. |
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