Structure H Clathrate Hydrates in Methane−Halogenic Large Molecule Substance–Water Systems

Abstract

We investigated crystallographic variations in structure H (sH) hydrates hosting CH4 and halogenic large-molecule guest substances [halogenic large-molecule guest substance (LMGS)]. The three halogenic LMGSs, namely, chlorocyclohexane (ClCH), bromocyclohexane (BrCH), and iodocyclohexane (ICH), share a common molecular structure (X-cyclohexane). The lattice constants along a and c axes of sH hydrates hosting X-cyclohexane increased with increasing molecular size: ClCH < BrCH < ICH. The c lattice constant was especially dependent on the molecular size, possibly because X-cyclohexanes align along the longitudinal direction of the 51268 cage, which coincides with the c-axis. Raman spectroscopy revealed that LMGSs altered the surroundings of CH4 molecules in the 512 and 435663 cages. As the crystal size increases, CH4 molecules encounter more attractive (less repulsive) guest–host interactions. The wavenumber shifts of the C–H vibrations of CH4 in the 512 and 435663 cages increased with temperature and were slightly greater in the 435663 cages than in the 512 cages. Different thermal responses between the 512 and 435663 cages may be caused by anisotropic lattice expansion of the sH hydrates. Finally, the phase stabilities of the sH (CH4 and X-cyclohexane) hydrates were evaluated by an isochoric method. The pressure region of equilibrium pressure–temperature conditions was lower in the sH (CH4 and X-cyclohexane) hydrates than in the pure CH4 hydrate system. Moreover, the temperature region of the equilibrium pressure–temperature conditions increased in the order ICH < ClCH < BrCH. The dissociation enthalpies of the sH (CH4 + ClCH) and sH (CH4 + ICH) hydrates were estimated as 380 kJ/mol–1.