Dielectric constant of silica gel activated at different temperatures

Abstract

The dielectric constant ε(25°C, 1 Kc/s) of a chromatographic gel was determined after activation in vacuo at temperatures from 150 to 1015°C. The specific surface was evaluated from nitrogen sorption isotherms (–195°C). The total hydroxyl content, which may have included some intra-globular hydroxyl groups, in addition to “perturbed” and “free” surface hydroxyls, was determined by thermogravimetric analysis. For activation temperatures up to near the Tamman temperature (755°C) the composite polarization per gram of silica gel was a linear function of the total hydroxyl concentration, C_OH(mol g^–1). Above this temperature, the polarization was independent of C_OH. These results can be explained on the basis of a polarization contribution from “perturbed” hydroxyl groups (where the hydroxyls can rotate from one hydrogen-bonded position to another), that from the isolated “free” surface hydroxyls remaining after activation above 755°C being negligible. Additional measurements of ε were made on silica gel (activated at 664°C) at –85 and –195°C. The increase in ε observed with decrease in temperature confirmed that part of the polarization of the system was contributed by surface hydroxyls constrained to rotate within cones about the Si—O axes.