| 초록 |
We report an extraordinary structural transition accompanying the occurrence of more than two coexisting clathrate hydrate phases in the mixed ionic (CH4 + tetramethylammonium hydroxide (Me4NOH)) hydrate system. The changing structure of ionic (CH4 + Me4NOH) hydrate was investigated by NMR spectroscopy, and lattice structure information was obtained by assigning the structure-specific chemical shifts from 13C NMR spectra. In addition, the structures of the ionic hydrates identified from NMR spectroscopy were confirmed by using powder X-ray diffraction (PXRD). From the 13C NMR spectra, it is confirmed that structure-I (sI) and structure-II (sII) hydrates are dominant when water concentration is more than 16.0 mol H2O per 1.0 mol Me4NOH (16.0 H2O). Significantly, quiet small signals from CH4 in large cages of sI and sII hydrates, compared to ideal distributions of CH4 in sI and sII cages, imply that Me4N+ is enclosed in both sI-L and sII-L at the higher degree of hydration, and sII preference of cationic guest can be also confirmed by the 1H NMR spectra. PXRD patterns and 13C NMR spectra reveal the complex structure transition of ionic (CH4 + Me4NOH) clathrate hydrate as a function of water concentration. |
