Dielectric behaviour of cyclohexane and benzene sorbed on a microporous silica gel

Abstract

Sorption isotherms have been determined for nitrogen (–195°), cyclohexane (25°) and benzene (25°) on a commercial silica gel. From their analysis, in addition to a pore system of 19 Å mean radius, micropores existed, which were penetrated by nitrogen and benzene molecules but not by cyclohexane molecules. From a nitrogen t plot it was estimated that micropores of width <7 Å contributed about one-third of the total surface area.

Dielectric isotherms (25°) for cyclohexane and benzene were reversible and frequency independent from 1 Kc/sec to 1 Mc/sec. That for cyclohexane was linear throughout the sorption range to saturation, indicating weak interaction between the silica gel surface and the adsorbed non-polar monolayer. The benzene dielectric isotherm comprised two linear regions. The initial region, of lower slope, is associated with micropore filling or “persorption”, with interaction occurring between the π-electrons of an adsorbed benzene molecule and silanol groups from opposite walls of a micropore. The second linear region corresponds to sorption in the wider pore system, although many of the pores are still narrow enough to fill predominantly by multilayer adsorption. On heating the silica gel to 440°in vacuo the micropore system was diminished, causing a decrease in the sorption of benzene, although not of nitrogen. The benzene dielectric isotherm again showed an initial linear region of lower slope than the second, but the “break” occurred at a lower (micropore) volume than with the unheated gel. A third linear region was detected, of lower slope than the second, this break occurring at the start of irreversible capillary condensation which was more pronounced in the calcined gel.