ИСТИНА

Bulk nanoporous materials can be used to develop catalysts, chemical reactors, fuel cells, selective adsorbents, filters, sensors, implants etc. Study of adsorption, phase behavior including near critical phenomena, molecular dynamics and interaction inside the pores is a subject of both fundamental and practical interest [1 and Refs inside]. At filling small mesopores and micropores with a molecular fluid, the fraction of molecules directly interacting with the pore walls is comparable with the total amount of the fluid inside pores. In this case, the fluid phase structure and the dynamics of equilibrium state are quite complex. Phase transformations of the fluid lead to corresponding changes of molecular vibrational spectra. In the case of the transparent substrates, nonlinear optical methods of vibrational spectroscopy, characterized by high spectral, temporal, spatial resolution and sensitivity, can be very informative. Besides, they are applicable to various porous networks with different porosity, pore size and shape, degree of interconnection and disordering. There are a number of optically transparent nanoporous materials such as nanoporous glass and other silicates, aerogels, cross-linked polymers, zeolites, which cover a wide range of various porous network types. That opens great opportunities for studying the phenomena occurring under nanoconfinement using nonlinear optical techniques. CARS spectroscopy has been efficiently applied to diagnostics of carbon dioxide phase behavior inside nanoporous glasses with pore diameters of several nanometers [2]. The phase transition between the gaseous and liquid-like states inside the nanopores at room and subcritical temperatures is accompanied by visual transformation of vibrational spectra caused by the appearance or disappearance of corresponding spectral contributions [3]. The spectrum of molecules forming a submonolayer surface coverage was identified and characterized [1]. In the present paper, the developed approach is used to study the appearance of condensed phase of carbon dioxide in nanoporous glass Vycor with pore radius of 2 nm. The density of the condensed phase is characterized by the Raman shift and compared to that in the bulk liquid. Particular attention is paid to the distinction between the two dense states inside the pores: the surface-adsorbed layers and the liquid-like fluid. The work is supported by Russian Foundation for Basic Research (16-02-01022) and by 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).