Raman spectroscopic observations of methane-hydrate formation and hydrophobic hydration around methane molecules in solution

Abstract

To reveal the hydrophobic hydration process of methane molecules dissolved in water, Raman spectra of dissolved methane (CH4) in water were measured under various conditions. The conditions include water saturated with CH4 gas, water-CH4 solution with CH4 hydrate crystals in equilibrium, and also during hydrate decomposition. The symmetric C-H stretching mode of the CH4 molecule in water is a single peak at 2910 cm-1 with a half-width of approximately 5 cm-1. These results indicate that the size of the space for the CH4 molecules, called the hydration shell, is between the large and small cages of the hydrate crystal, but it has a broad size distribution. To better understand the CH4-molecule vibrations, its spectrum in water was compared with its spectra in liquid carbon dioxide (CO2) and in liquid ethane (C2H6). These spectra were very similar to those observed in water, except that the peak widths were sharper than those in water. This suggests that the broadening of the shell-size distribution is due to the way the CH4 molecules affect the hydration shell. On the other hand, when the system included hydrate crystals, a double CH4 peak arose due to structure in the aqueous solution near the crystal surface. This indicates that a cage-like structure can exist in the water phase. Compared with the decomposition experiment, the cage-like structure is likely due to the presence of the bulk hydrate crystal and is different from the hydration shell for the dissolved CH4 molecules.