Hydroxyl groups on silica surface

Infrared (IR) gravimetric adsorption data show that the major adsorption sites on a silica surface are surface hydroxyl groups. Depending upon the temperature of pretreatment these may be either ‘freely vibrating’ or hydrogen bonded to each other. The adsorptive properties of each type of group are very different. Thus, whereas water will adsorb preferentially on the hydrogen-bonded hydroxyl groups, compounds containing lone-pair electrons adsorb preferentially on the freely vibrating hydroxyl groups. This physical adsorption in turn controls the order of chemical interaction with the groups. The freely vibrating hydroxyl groups are evidenced by a single, sharp IR band at 3747 cm^?1. However, both their physical adsorption and chemical reactivity show that they themselves are composed of two types of groups. This is clearly demonstrated by their chemical reactions with silane coupling agents.Small amounts of impurity have a gross effect on the chemical reactivity of the surface. The presence of small amounts of boron on the silica surface not only gives rise to specific adsorption sites of the Lewis acid type, but also alters the reactivity of the silanol group which (spectroscopically) appears unchanged. Thus, the kinetic order of the reaction of dichlorodimethylsilane with the surface hydroxyl groups is reduced from 1.6 order to 1.0 order after the surface has been purposely unpurified by the introduction of boron.The implications of these data in terms of the glass surface are discussed.