Dielectric behaviour of cyclohexane and benzene sorbed on aluminas of known specific surface

Abstract

The nitrogen isotherm (–195.3°) and standard B.E.T. specific surface of a partially hydrated alumina (A) has been determined. These results confirm that the break in the respective dielectric isotherms determined previously with this adsorbent, for water, methanol, ethanol, cyclohexanol and benzene, is associated with the completion of a close-packed molecular monolayer, which is immediately followed by capillary condensation within the smaller pores. Sorption data for cyclohexane (25°) indicates that these molecules are, in contrast, loosely packed in the adsorbed monolayer, apparently occupying an area of 60 ± 3 Ų. The dielectric isotherm is reversible, linear and frequency independent (1 kc/sec-1 Mc/sec) to near saturation, confirming weak adsorbent + adsorbate interaction.

Dielectric curves (change in dielectric constant against amount adsorbed) for benzene on alumina A, a hydrated, and a calcined (440°) sample, approximately coincide, and show little deviation from linearity up to saturation. The dielectric behaviour within the capillary condensation region is independent of pore size, and within the monolayer sorption region appears little dependent on the nature of the alumina surface. Further evidence from mean pore radius calculations confirms that molecules of benzene, cyclohexane and cyclohexanol are adsorbed in the monolayer with the plane of the ring parallel to the alumina surface.