Abstract
The effect of substituting nitrogen heteroatoms in the cyclohexane ring of methylcyclohexane on the structure and guest dynamics of structure H (sH) clathrate hydrates with methane help gases are studied through experimental synthesis, powder X-ray diffraction (PXRD) measurements, and classical molecular dynamics simulation of methylcyclohexane and 1-methylpiperidine. The PXRD measurements were performed for temperatures in the range of 138 to 183 K, and the a axis and c axis lattice constants were determined in this temperature range. The PXRD results show the different thermal expansivity of lattice constants in both sH hydrate cages. Simulations on methylcyclohexane and 1-methylpiperidine are performed, and the effects of methane cage occupancy on the lattice constants were studied by simulations with 100% and 80% of the small and medium cages occupied by methane. The sH phases do not expand isotropically, and the a and c lattice constants can vary over a range of 0.015 Å and 0.06 Å, respectively, depending on the large cage guest and methane occupancy. Hydrogen bonding between the 1-methylpiperidine nitrogen atom and cage water in the sH hydrate are not observed in the simulations, and differences in behavior of the two sH hydrates are likely related to the differences in geometry of the large guests and occupancies of methane in the small and medium sH cages.
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