hydrolysis. The drying time and temperature also affect the pore size of the gel as well as

the shape of the gel. For example, the longer the time to dry, the more Ostwald ripening

occurs and the less likely the gel will crack, thereby producing a larger pore size than a

gel of the same composition dried in short period of time (less than 48 hours). In

summary, there are many parameters in the production of a gel that will greatly affect the

final product's characteristics. It is these differences in silicas produced by various

preparation methods that make the comparison of data found in the literature a very

difficult process.

2.5.2 Surface Chemistry and Adsorption Characteristics

 The surface of silica consists of two types of functional groups: silanol groups (Si

- O H) and siloxane groups (Si O Si). The silanol groups are the locale of activity

for any process taking place on the surface, while siloxane sites are considered

nonreactive (Unger, 1979). Porous amorphous silica contains three types of silanols on its

surface: isolated, geminal, and vicinal (or associated). Figure 2-7 shows the distinction

between these groups. The unequal distribution of the silanols in the sililca matrix,

resulting from irregular packing of macromolecules as well as incomplete condensation,

results in a heterogeneous surface (non-uniformity in the dispersion of silanol groups) for

synthesized silica (El Shafei, 2000).

 The various silanols can have different adsorption activity and current knowledge

indicates that the isolated silanols are the more reactive species (Nawrocki, 1997; El

Shafei, 2000). With increasing temperature of heat treatment, the silica surface becomes

hydrophobic due to the condensation of surface hydroxyls (dehydroxylation) and the

formation of siloxane bridges (see Figure 2-8). This increases the ratio of isolated sites

versus other silanol groups on the surface and therefore increases adsorption.