A. A. Valeev

CARS diagnostics of molecular media under nanoporous confinement

The CARS spectroscopy is used for the diagnostics of carbon dioxide in a nanoporous glass at temperatures ranging from room temperature (20.5°C) to the subcritical temperature (30.5°C) in the pressure range below the saturated-vapor pressure. The contributions of the gas-phase molecules, the molecular layer adsorbed from the gas phase on the pore surface, the condensed liquid-like phase, and the liquid interface in the vicinity of the pore surface can be selected using the analysis of the nonlinear spectral response. The spectral behavior of the carbon dioxide confined in nanopores at the subcritical temperature indicates a state that is similar to the supercritical fluid. This corresponds to a low-temperature shift of the critical point of the medium confined in nanopores.

Phase Behavior of the Molecular Medium in Nanopores and Vibrational Spectra Structure Transformation

A model description of molecular medium spectra during phase transitions in cylindrical nanopores is developed based on the thermodynamic concept of surface adsorption and capillary condensation. Good agreement of the calculated and experimental spectra measured during carbon dioxide adsorption and condensation in pores of nanoporous Vycor glass with a diameter of 2 nm is obtained. The constructed model connects the behavior of the medium spectra with the characteristics of the porous structure of a material and can be used for their determination based on spectroscopic data.

Broadening of vibrational spectra of carbon dioxide upon absorption and condensation in nanopores

Coherent anti-Stokes Raman Spectroscopy (CARS) has been used to study the vibrational Q-branch with the frequency of 1388 cm−1 of the ν1 mode of carbon dioxide molecules filling a sample made of nanopore glass at room temperature (20.5°C). The measurements were carried out in a gas cell at pressures approaching saturation Psat. When pressure was increased above 0.8 Psat, in addition to the spectral component due to the gaseous phase molecules, the CARS spectra featured a component due to the molecules adsorbed on the pore walls. Simulation of spectra taking the interference of these two contributions into account enabled the estimation of the broadening of the vibrational molecular spectra in the adsorbed layer. The spectral width of the component due to the adsorbed molecules was nearly a factor of two times larger than that of molecules in the bulk liquid phase. At pressures above 0.94 Psat, the spectral width of the component due to the adsorbed molecules decreased to values close to those measured in the bulk liquid phase, which corresponds to the condensation of molecules in nanopores.

Broadening Peculiarities of Vibrational Bands in the Spectrum of Carbon Dioxide Close to the Critical Temperature

Carbon dioxide Fermi doublet 1388/1285 cm−1Q-band broadenings and shifts measured using coherent anti-Stokes Raman spectroscopy are presented. Measurements were performed over a wide density range (0.1ρc < ρ < 1.9ρc) during compression in the gaseous and condensed states at temperatures close to critical (the reduced temperature values were Tr = 0.995, 1.000, and 1.006). At densities above the ρc critical value, the width of Q-bands did not increase as the density grew, and the low-frequency Q band considerably narrowed up to the density value 1.7ρc. The main reason for this anomalous behavior was progressing narrowing of the spectral contribution caused by the special features of rotational exchange in the condensed state and not related directly to the closeness to the critical point. The refined critical broadening value was about 10% of the width for the high-frequency Q-band and 15% of the width for the low-frequency Q-band.

Spectral Characteristics of Subcritical Carbon Dioxide in Nanopores

Coherent anti-Stokes Raman scattering spectra measured within the Q-branch of the vibrational transition ν1 are used to gain insights into the state of carbon dioxide molecules in nanopores of Vycor™ glass at room temperature (20.5°C) and a subcritical temperature of 30.5°C and gas pressures up to the saturation point Psat for each temperature. Along with the main spectral component, belonging to gaseous CO2 molecules, the spectra recorded at pressures close to Psat feature a second (low-frequency) component. The second component is associated with the contribution from the CO2 molecules trapped inside pores. A spectral deconvolution with account for the interference of these two bands makes it possible to estimate the spectral characteristics of the second (low-frequency) component at each temperature. At 20.5°C, the bandwidth of the low-frequency component decreases with CO2 pressure, a behavior that can be explained by the transition of CO2 from the adsorbed to the condensed state in the pore. At the subcritical temperature of 30.5°C, the spectral width of the second component is pressure-independent and close to the value measured in the bulk of the supercritical fluid, a result likely associated with a low-temperature shift of the critical point of the substance trapped in nanopores.