Narrowing of the vibrational spectrum under compression of liquid carbon dioxide

Bekin

Morozov

2018

Coherent anti‐Stokes Raman scattering is applied to distinguish dense phases of carbon dioxide adsorbed in mesoporous glass Vycor with pores of 2 nm in radius. Each of the phases is identified by their Raman shifts corresponding to the CO2 band at 1388 cm−1. The results show that the first layer adsorbed on the pore wall and the polymolecular layers adsorbed upon it can be spectroscopically distinguished from each other. Moreover, the spectrum of the polymolecular layers, usually considered as liquid‐like, is found to be noticeably shifted from that of the liquid phase, making these two similar phases also distinguishable. After the onset of condensation, the spectrum of appearing liquid phase is found to be at least threefold broader than that of the bulk liquid. As pressure is increased, it narrows down to the value in the bulk liquid. Observed narrowing reflects size enlargement of liquid clusters, which being initially of nanometer scale in all three dimensions grow up and merge together with pressure increase, finally turning into a single “infinite” cluster, that is, bulk liquid. The results show that Raman‐based methods can help to detect and identify the phases of adsorbate confined in transparent nanoporous hosts. Needing no a priori knowledge about the characteristics of pores and not relying on any modeling or simulation of adsorption, the presented approach is promising to characterize the adsorption mechanisms in mesoporous and microporous materials.

Section: Resultssupporting

confidence: 57%

Spectroscopic characterization of adsorbate confined in small mesopores: Distinction of first surface‐adsorbed layer, polymolecular layers, and liquid clusters

Bekin

Morozov

2018

“…In this case, a third resonant contribution appears in the Eqn . The linewidth of the spectral contribution of the condensed phase was assumed to be 1.55 cm −1 , similar to the values measured in bulk liquid (Fig. ) and in nanopores at temperatures far below the critical value .…”

Section: Resultsmentioning

confidence: 93%

CARS diagnostics of fluid phase behavior in small mesopores at near‐critical temperatures

Morozov

2013

Self Cite

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is applied to investigate the fluid phase behavior in small mesopores at bulk sub-and supercritical temperatures. CARS spectra of the Q-branch (1388 cm À1 ) of carbon dioxide were measured at isothermal compression in pores of two nanoporous glass samples with pore radii of 2 and 3.5 nm. The spectral transformations with pressure of the fluid indicate the formation of a condensed phase in the volume of pores that clearly demonstrates the shift of the critical point. The spectrum of carbon dioxide condensed in the wider pores corresponds approximately to the critical density in bulk. In the narrower pores, the spectrum is noticeably shifted towards the lower wavenumbers that correspond to higher density of condensed fluid.

“…Calculations of spectral profiles were performed according to the Eqns (8) and (9). The line width and shift parameters were taken from experimental results reported previously [21] : the coefficient of the homogeneous broadening in the gas is γ = 7.5 × 10 −3 cm −1 /Amagat, and the shift coefficient is δ = −7 × 10 −3 cm −1 /Amagat.…”

Section: Results and Analysismentioning

confidence: 99%

“…In the nanopores, the condensed state was assumed to be similar to that in the bulk liquid. Thus, the line width and Raman shift values were selected as 1.55 and 1386.0 cm −1 , [9] respectively. These values correspond to the bulk liquid with a minimum density at the given temperature [16] (at 20.5 • C the density is equal to ∼390 Amagat).…”

Section: Results and Analysismentioning

confidence: 99%

“…At the same time, at pressures relatively far below saturation, the low wavenumber peak line width was ∼3-4 cm −1 , which is about twice the value of 1.55 cm −1 for the bulk liquid. [9] The large line width value was attributed to the layers adsorbed on the pore walls. As the pressure approached the saturation value, the line width dropped down to the bulk liquid value.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

CARS diagnostics of fluid adsorption and condensation in small mesopores

Andreeva

Баграташвили

et al. 2011

Self Cite

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is applied to diagnostics of phase behavior of a fluid in pores of nanoporous glasses. Samples with mean pore radii of 2 and 3.5 nm were filled with compressed carbon dioxide at near-room temperatures. CARS spectra of the 1388 cm −1 Q-branch were measured at isothermal compressing in a wide pressure range including the transition from gaseous to condensed state. The spectra show specific transformations caused by fluid adsorption and condensation in nanopores. We have carried out calculations of the spectral profiles based on the phase behavior of carbon dioxide in cylindrical glass nanopores. Phase behavior modeling was performed using thermodynamic concepts of surface adsorption and capillary condensation. A good agreement between experimental spectra and calculations was obtained. The potential of CARS technique for the diagnostics of fluid phase behavior in pores and for the characterization of nanoporous host structure is discussed.