The Characterization of the Hydroxyl Surface of Silica Gel

Abstract

The properties of silica gel were studied as a function of the hydrothermal treatment of hydrogel by means of the nitrogen-adsorption isotherm, the mechanical strength, thermodilatometry, and infrared spectroscopy. As a result, the number of contact points between primary particles of silica was found to decrease, and closed pores were found to be produced, by this treatment. The analysis of the infrared absorption spectra of the OH groups of this material showed that there are three kinds of OH groups in silica gel; (1) free OH groups on the outer surface of primary particles, which can adsorb molecules easily, (2) weakly perturbed OH groups inside closed pores, and (3) strongly hydrogen-bonded OH groups on the surface around the contact points. The relative concentration of these OH groups changes drastically upon the hydrothermal treatment; this is in accord with the change in the macroscopic properties mentioned above.