Effect of Nonspherical Encapsulated Guests on the Volumetric Behavior of Structure H Clathrate Hydrates

Since its first discovery, technologies utilizing the structure H (sH) clathrate hydrates have been proposed. To develop the technology, understanding the mechanism of thermophysical property manifestation would be of fundamental importance. It is reported in the literature that the molecular properties of guest molecules had a significant influence on the crystal structure and the resulting thermophysical properties of the sH hydrate. In the present study, the effect of linear N2 help-gas molecule on the crystal structure was experimentally and computationally investigated to focus on the shape of the guest molecule in the sH binary clathrate with neohexane. Powder X-ray diffraction (PXRD) measurements were performed in temperatures from 93 to 183 K. The lattice constant values on a and c axes, the cage occupancies of small 512 and 435663 cages, and guest distributions in the host cages were determined. Parrinello–Rahman molecular dynamics (MD) simulations were performed on the sH clathrate hydrate phase. The simulated values of the lattice constants indicated the same tendency as the experimental results. The MD simulations provided detailed descriptions of the molecular motions of guest molecules. On the basis of these results, the influence of a small guest molecule on the crystal structure of sH hydrate is discussed