Characterization of surface OH groups on porous glass.

Abstract

The surface hydroxyl groups on porous glasses pretreated at various temperatures was determined by the successive-ignition-loss method and by means of active hydrogen analyses, and their hydroxyl groups have been classified into two types (isolated and gem type OH groups). The surface of the porous glass consists of three types of OH groups, isolated, gem type and H-bond OH groups. The isolated OH groups were stable up to 500°C, but the elimination of these groups took place gradually above 500°C. On the other hand, the gem type and H-bond OH groups were removed rapidly below 600°C and disappeared above 800°C. The surface of porous glass pretreated at 800°C for 4h contained only isolated OH groups, and the concentration of the isolated OH groups was found to be about 0.7OH's/100Å2. For water vapor adsorption, the surface OH groups play an important role as adsorption sites, and water molecules are adsorbed more strongly on gem type or H-bond OH groups than on isolated OH groups. In porous glass, the ignition-loss method was inadequate to determine the amount of surface OH groups, since water molecules diffused from inside the glass to its surface during heat treatment.