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In Situ Observations of Methane Hydrate Formation Mechanisms by Raman Spectroscopy

Abstract: We compared time‐resolved Raman spectroscopic measurements of CH4‐saturated aqueous solution with CH4 hydrate crystal formed from the same solution. At 287.1 K and 6.6 MPa the C‐H stretching mode for CH4 molecules in the solution (νsol= 2911.7 cm−1) was intermediate between that in the large cage (νL= 2905.1 cm−1) and that in the small cage (νS= 2914.1 cm−1), thus indicating an intermediate molecular environment. Comparison of these results with previous work on the CO2‐H2O system indicated that water molecules in the type‐I small‐cage‐like structure surrounded CH4 molecules in the solution. Furthermore, the free energy barrier for CH4 hydrate nucleation is likely to be formation of the large cage structure, which is probably larger than that for CO2 hydrate formation.