ABSTRACT
Si -OH. The latter is the result of incomplete condensation during the polymerization process through which the massive structure is formed and/or the tendency of the surface oxygens to form two bonds as other atoms in the interior. The proton is the most suitable candidate to be used by surface oxygens to complete their bonding, which is supplied by water that is normally present in the media around the surface (during processing). Hydrous silica is therefore that formed in presence of water (liquid or vapor), and it is characterized by high population of surface hydroxyls. Anhydrous silicas are formed at high temperatures and recovered from the gas phase in dry atmosphere; its surface appears to accommodate a very small number ofhydroxyls if water is omitted. Hydrous silica is the most important when adsorption is considered, since these surface hydroxyls represent the key of activity of silica in any process taking place at the surface. The least important property of silica as a desiccant relies completely upon silica hydroxylation.
