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

摘要

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 (CICH), 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: CICH < BrCH < ICH. The c lattice constant was especially dependent on the molecular size, possibly because X-cyclohexanes align along the longitudinal direction of the 5(12)6(8) cage, which coincides with the c-axis. Raman spectroscopy revealed that LMGSs altered the surroundings of CH4 molecules in the 5(12) and 4(3)5(6)6(3) 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 5(12) and 4(3)5(6)6(3) cages increased with temperature and were slightly greater in the 4(3)5(6)6(3) cages than in the 5(12) cages. Different thermal responses between the 5(12) and 4(3)5(6)6(3) 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).