Surface Chemistry of Silicas The Silanol Groups

The surface chemistry of silicas is dominated by the surface hydroxyl groups, or silanols (Si-O-H). The silanol groups participate in adsorption (for water and other compounds, including organic compounds) as well as the chemical modification of silica surfaces. The subject of surface silanols has been studied extensively over the past seven decades and has been reviewed (e.g., Her, 1979 Unger, 1979 Zhuravlev, 1993 Bergna, 1994 Vansant et al 1995). 

Hydrogen bonding is the most important type of bonding for adsorption of water as well as other componnds. For water to adsorb on the first layer of silanol, it has been shown that water sits oxygen down on the SiOH gronp (Klier and Zettlemoyer, 1977). As more water molecules adsorb, hydrogen-bonded clnsters form. The heats of adsorption are (Her, 1979 Zhuravlev, 1993) the following  

The value of 10.5 kcal/mol is very close to the latent heat of condensation of water (10.8 kcal/mol). 

The stretching frequency band for the isolated OH groups is a very sharp one. AH other bands are broad. This sharp band appears only after dehydration, for example, 200-400 °C in vacuo. 

The density of the OH groups on the surface is referred to as the silanol number, Q-oH, expressed in OH groups per square nanometer. The silanol number can be determined by deuterium exchange (Kiselev and Lygin, 1975 Zhuravlev, 1993) or 

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