Issue 33, 2023

Direct observation of pressure-induced amorphization of methane/ethane hydrates using Raman and infrared spectroscopy

Abstract

The pressure-induced amorphization (PIA) of ice and clathrate hydrates occurs at temperatures significantly below their melting and decomposition points. The PIA of type I clathrate hydrates containing methane and ethane as guest molecules was investigated using Raman and infrared (IR) spectroscopy. With isothermal compression at 100 K, methane hydrate (MH) underwent PIA at 2ā€“3.5 GPa, whereas ethane hydrate (EH) underwent PIA at 4.0ā€“5.5 GPa. The type I clathrate structure consists of small (512) and large (51262) cages. The Raman results revealed that the collapsed small and large cages in the amorphous forms of MH and EH were not distinguishable. The collapsed cages, including the methane and ethane molecules, were similar to the small and large cages, respectively. Their water networks were folded or expanded during the PIA process so that the cavity sizes of the collapsed cages were compatible with those of the guest molecules. Peaks in the IR spectra of crystalline MH assignable to the ro-vibrational transition of methane in large cages were observed in the Cā€“H stretching wavenumber region below 40 K. The ro-vibrational IR band disappeared after amorphization, suggesting that the rotational motion of the methane molecule in the large cage was frozen by the collapse, as reported in previous dielectric spectroscopic and simulation studies. This study contributes to a better understanding of the changes in the local structure around guest molecules during PIA and the dynamics of the guest molecules.

Graphical abstract: Direct observation of pressure-induced amorphization of methane/ethane hydrates using Raman and infrared spectroscopy

Supplementary files

Article information

Article type
Paper
Submitted
01 Jul 2023
Accepted
07 Aug 2023
First published
07 Aug 2023

Phys. Chem. Chem. Phys., 2023,25, 22161-22170

Direct observation of pressure-induced amorphization of methane/ethane hydrates using Raman and infrared spectroscopy

N. Noguchi, Y. Shiraishi, M. Kageyama, Y. Yokoi, S. Kurohama, N. Okada and H. Okamura, Phys. Chem. Chem. Phys., 2023, 25, 22161 DOI: 10.1039/D3CP03096B

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