SURFACE HYDROXYLATION OF SILICA

The surface hydroxylation of silica gels and powders has been studied by determining the stoichiometry of the reactions of SiMe2Cl2, TiCl4, and BCl3 with the surface hydroxyl groups. The results demonstrate that the fully hydroxylated silica surfaces studied carry two distinct types of surface hydroxyl sites, which are distributed as follows: 1.4 ± 0.1 single surface hydroxyls (type A sites), and about 3.2 ± 0.1 interacting hydroxyl groups arranged in pairs (1.6 ± 0.1 type B sites). On heating the silicas above ambient temperatures, in vacuo, the type B sites are progressively removed. By correlating this result with earlier studies, it is concluded that at temperatures of 500 ± 50°, in vacuo, virtually all the type B sites are eliminated. However, the type A site concentration remains constant at evacuation temperatures up to 600 ± 50° at around 1.4 ± 0.1 A sites/100 Å2. These conclusions suggest that the silica surface corresponds to an array of different crystal planes, some of which contain OH groups at relatively large interhydroxyl spacings and others containing OH groups held in such a way as to promote interhydroxyl hydrogen bonding.