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

• Published in: Canadian Journal of Physics
• Main Authors: Uchida, T, Okabe, R, Gohara, K, Mae, S, Seo, Y, Lee, H, Takeya, S, Nagao, J, Ebinuma, T, Narita, H
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2003
• Subjects: Methane hydrate
• Online Access: http://dx.doi.org/10.1139/p03-019; http://www.nrcresearchpress.com/doi/pdf/10.1139/p03-019

To reveal the hydrophobic hydration process of methane molecules dissolved in water, Raman spectra of dissolved methane (CH 4 ) in water were measured under various conditions. The conditions include water saturated with CH 4 gas, water–CH 4 solution with CH 4 hydrate crystals in equilibrium, and also during hydrate decomposition. The symmetric C–H stretching mode of the CH 4 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 CH 4 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 CH 4 -molecule vibrations, its spectrum in water was compared with its spectra in liquid carbon dioxide (CO 2 ) and in liquid ethane (C 2 H 6 ). 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 CH 4 molecules affect the hydration shell. On the other hand, when the system included hydrate crystals, a double CH 4 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 CH 4 molecules. PACS Nos.: 81.10Dn, 33.20Fb