Modification of the porous structure of silica gel on purification and the associated change in the dielectric behaviour of adsorbed benzene

Abstract

A commercial silica gel, with micropores (width < 7 Å) contribution over one-quarter of the specific surface, was purified by nitric acid treatment. Analysis of nitrogen (–195°) and benzene (25°) sorption isotherms showed that purification eliminated half the micropores, promoted sorption hysteresis, and increased the mean pore radius of the wider-pore system.

The dielectric isotherm (25°, 1 Mc/sec) for benzene on the purified gel showed only a short curved initial region, associated with persorption in micropores, in contrast to the extensive linear region exhibited by the original gel. The following linear reversible region, associated with monolayer formation in wider pores, extended to near saturation with the original gel, but on purification, a second reversible linear region of lower slope occurred at monolayer completion. The latter region is associated with both reversible and irreversible capillary condensation, the former forming saddle-shaped menisci at points of contact between the primary silica particles, and the latter occurring in the pore-spaces after coalescence of the saddle-shaped menisci. This region was not detected (with any certainty) in the dielectric isotherm given by the impure gel, where layer adsorption alone was sufficient to fill the majority of the pores, and little irreversible capillary condensation occurred.