New X-ray method for investigation of mass transport in disperse porous systems

Abstract

The new X-ray method, based on small-angle X-ray scattering (SAXS) data, was proposed for studying mass transport in porous systems. It is based on the dependence of SAXS intensity on the difference in electron densities for pores and matrix. During the filling of pore space (by vapour, liquids) the electron density differences and, accordingly, the SAXS intensities decrease. The reverse effect takes place during the refilling of pores. Using this dependence, we succeeded in calculating the scattering for filled pores and determining the size distributions in pore volumes. It enabled us to connect different stages of the adsorption–desorption process to the sizes of filled pores. The SAXS method was used to investigate systematically adsorption–desorption of a number of fluids (water, benzene, glycerol, trixylylphosphate) in carbon adsorbents. It was shown that different characters of these processes depend on the liquid structure and sizes of its molecules, and on the peculiarity of micro- and mesoporous structures of adsorbents. An analytical expression was obtained, which permitted us to describe the kinetics of desorption using SAXS data. The sizes of micropores, which were inaccessible for large molecules of adsorbed fluids were estimated. It was shown that there are micropores, into which whole molecules could not penetrate, but only their fragments.