Sorption and dielectric behaviour of benzene on silica gel

Abstract

Benzene sorption and dielectric isotherms (1 Mc/sec, 25°C) redetermined on an acid-purified “chromatographic” silica gel after 6 months storage showed evidence of ageing. The dielectric isotherm, as before, comprised a short initial curved region followed by three distinct linear regions. The initial region and first and second linear regions are again associated with (i) reversible filling of a small number of micropores, (ii) reversible monolayer adsorption and (iii) irreversible capillary condensation within pores. The third linear region is now identified with (iv), reversible sorption of “bulk” benzene (in surface films and in coarse V-shaped pores) at relative pressures above that corresponding to the upper point of closure of the sorption hysteresis loop.

A quantitative treatment, in which the surface hydroxyl groups (considered to be rotating about the Si—O bonds) are treated as a separate phase, is applied to these silica gel + benzene systems. Values of the dielectric constant of adsorbed benzene calculated from the first and third linear regions of each dielectric isotherm are in good agreement with values for solid and liquid benzene, respectively. The decrease in slope in the dielectric isotherm associated with capillary condensation in pores (second linear region) is considered to be due to the dimunition of the orientation polarization contributed by the surface hydroxyl groups within full pores.