Water vapour adsorption and contrast-modified SAXS in microporous polymer-based carbons of different surface chemistry

Abstract

The adsorption of water vapour on highly microporous activated carbons with different surface chemistry is investigated by small angle X-ray scattering (SAXS) as well as by adsorption isotherms. The water changes the intensity of the SAXS in a way that depends on how the pores are filled. With wetting liquids such as hexane, a pseudo binary model can be assumed in which pore-filling in reciprocal space q is described by a density function p(q). For water, clusters develop, even in the most oxidized carbon, creating a fully ternary system. In the Porod scattering region, however, the final slope is insensitive to the liquid-vapour interfaces. In this region, for the less oxidized samples, p(q) shows reasonable agreement with the adsorption isotherms. At low relative pressure P/P0, however, the SAXS results indicate a small degree of filling (about 10%) that is not reflected in the isotherms. The highly oxidized sample attains a degree of filling of about 70% that, unlike the corresponding isotherm, is constant for P/P0>0. These differences may be due to kinetic effects and/or ageing, involving either redistribution of the water molecules or modifications of the surface groups.