High-pressure variable-temperature sum-frequency vibrational spectroscopy for in situ molecular probing of...

Here, we describe a high-pressure, variable-temperature sum-frequency vibrational spectroscopy (SFVS) apparatus designed for in situ, surface-specific measurements of aqueous interfaces during gas-hydrate formation. The apparatus integrates an ultra-clean gas-handling system, a sealed optical chamber with controlled pressure and temperature, and a broadband SFVS spectrometer with real-time reference normalization. The chamber operates from -25 to 60 °C and from ambient pressure to 10.0 MPa, allowing interfacial spectra to be recorded while the sample is driven across the methane-hydrate phase boundary. Using the methane/D2O interface as a representative system, we monitor the OD-stretch response of interfacial water during cooling under 8.0 MPa CH4. The temperature-dependent spectral evolution reveals the emergence of hydrate-like interfacial water ordering prior to macroscopic crystallization, demonstrating the ability of the apparatus to capture early molecular signatures of hydrate formation. We further show that the same platform can be extended to buried solid/liquid interfaces using the octadecyltrichlorosilane/water interface as a model system. These measurements establish a versatile experimental platform for correlating interfacial molecular structure with gas pressure, temperature, and surface chemistry, providing a route toward mechanistic studies of gas-hydrate nucleation, growth, and inhibition under controlled thermodynamic conditions.