geminal isolated single associated silanols
 H H H




 hydrogen bounded water molecules


Figure 2-7. Silanol groups on the silica surface (Legrand, 1998).

 At temperatures up to 450C, the removal of the hydroxyl groups does not result

in visible changes in structure, density, or specific surface area of the silica because the

rehydroxylation makes it possible to return to the initial state (Davydov, 2000).

Rehydroxylation of the silica surface by exposure to water vapor is reversible up to

600C. At temperatures above this, the rehydroxylation becomes a slow process or

remains incomplete (Unger et al., 2000). Also, there is a decrease in surface area as a

result of sintering at temperatures above 500C (Persello, 2000).


H- Oi H- -...-H
 Si Si


Si Si + HO


Figure 2-8. Representation of dehydroxylation (Burneau and Gallas, 1998).

 Pore size can be affected by the heat treatment of the silica, but is also a function

of the manufacturing process. The effect of pore size on adsorption is dependent on the

compound's characteristics, such as molecule size, shape, and functional groups

(Dabrowski, 2001; Goworek et al., 1997; Goworek et al., 1999). In general, the smaller

the pore, the larger the surface area and the larger the capacity for adsorption of