In situ observation of CH4–H2O binary system at high temperature and high pressure by diamond anvil cell and Raman spectroscopy

Phases and pressure–temperature (P–T) relationship of CH4–H2O binary system were observed in situ by diamond anvil cell (DAC) and Raman spectroscopy under various temperature and pressure. The system shows three states: (1) State I: methane in low density and immiscible with water at 75–150 °C and 80–189 MPa; (2) State II: methane in high density and immiscible with water at 175–220 °C and 265–303 MPa; and (3) State III: methane dissolved in water at above 225 °C and 303 MPa. Pressure drops by about 100 MPa in the system when the state transforms from II to III, indicating negative excess volume during the transition. Raman spectra of methane show discontinuity between State II and III, indicating multi-component interactions and system property variations during the phase transition.

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► In situ observation of the phases and P–T relation of CH4–H2O binary system by diamond anvil cell and Raman spectroscopy.
► There exist lower and higher density methanes at immiscible condition.
► There exists negative excess volume of methane and water at miscible condition at high temperature and pressure.