ИСТИНА

Nonlinear-optical spectroscopic techniques that employ signal from the molecules located inside the nanopores have promising potential in investigations of fluid behavior under nanoconfinement [1-3]. The following stages of adsorption are observed spectroscopically: monolayer formation, appearance of liquid-like domains, capillary condensation. Each of the phases is identified by its specific Raman shift. The relative mass fractions are estimated by the analysis of the spectral contributions of the phases. In the present work we apply coherent anti-Stokes Raman scattering (CARS) spectroscopy to investigate appearance of a liquid-like phase of carbon dioxide in mesoporous Vycor glass under isothermal compression. The results show that the liquid-like phase appears in the pores at pressure that is several times smaller than the capillary condensation pressure, at that the pore wall coverage is of the order or even less than a monolayer. The appearance of the liquid-like phase may be attributed to the layers adsorbed beyond the monolayer or to small liquid-like clusters arising in some specific areas of the porous network, which are characterized by strong confinement accompanied with high surface curvature. The density of the liquid-like layers (or clusters), estimated by the spectral shift, appears to be similar to that in the bulk liquid. An important point is that the analysis presented here does not imply any special knowledge about the porous system. Moreover, the experimental approach seems to have no limitations on characteristics of the pores (e.g. pore sizes, shapes, ordering, and interconnection), requiring only optical transparency of the material. The work is supported by Russian Foundation for Basic Research (16-02-01022) and by M.V. Lomonosov Moscow State University Program of Development. 1. V.G. Arakcheev, V.B. Morozov. Vibrational spectra of carbon dioxide adsorbed on nanopore walls at supermonolayer and submonolayer coverage. J. Raman Spectrosc., 45 (7), 501-506 (2014) 2. V. G. Arakcheev, A. A. Valeev, V. B. Morozov, and A. N. Olenin. CARS Diagnostics of Molecular Media under Nanoporous Confinement. Laser Physics, 18(12), 1451-1458 (2008). 3. O.V. Andreeva, V.G. Arakcheev, V.N. Bagratashvili, V.B. Morozov, V.K. Popov, A.A. Valeev. CARS diagnostics of fluid adsorption and condensation in small mesopores. J.Ram.Spectr., 42(9), 1747-1753 (2011).