CARS Diagnostics of Molecular Fluid Phase Behavior in Nanoporous Glassesстатья
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Дата последнего поиска статьи во внешних источниках: 4 марта 2022 г.
Аннотация:Coherent anti-Stokes Raman scattering (CARS) spectroscopy is applied to study the dense phases composition and detailed features of the phase state of carbon dioxide filling pores of nanoporous glass samples (2 and 3.5 nm mean pore radii) in a wide pressure range at sub-critical and supercritical temperatures. Each of the phases is identified by the spectral shift of the CO2 line corresponding to the Q-branch peak of the Raman band at 1388 cm−1. It is demonstrated that at sub-critical temperatures − 6 and − 13 °C the surface adsorbed monolayer and the polymolecular layers adsorbed above it can be spectroscopically distingushed from each other. Moreover, the spectrum of the polymolecular layers is found to be noticeably shifted from that of the liquid phase, making these two similar phases also well distinguishable. After the onset of condensation, the spectrum of the appearing liquid phase is at least threefold broader than that of the bulk liquid. As the pressure increases, it narrows down to the value of the bulk liquid. The observed narrowing reflects enlargement of the liquid clusters from primarily sub-nanometric and nanometric size ones with a significant fraction of molecules, constituting the outer border and interacting with the non-uniform environment, to finally infinite-size homogeneous liquid filling the pore network. The density of supercritical cabon dioxide confined in the pores at isochoric heating (from 30 to 68 °C) determined by the measurements of the Raman shift shows a significant (~ 30%) increase in the vicinity of the bulk critical temperature and asymptotically decreases at higher temperatures. The obtained results demonstrate that CARS diagnostics can be used for precise characterization of subcritical and supercritical carbon dioxide phase composition under the conditions of nanoconfinement.