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alcohol. As the number of siloxane bonds increases, the individual molecules join

together to create a silica network. See Equations 5-7 (Mauritz, 2000).


 I Hydrolysis
 -Si-OR + HOH -Si-OH + ROH (5)
 SReesterfication


 Water
 ICondensation 6)
 -Si-OH + -Si-OH i -Si- + HOH
 Hydrolysis |


 Alcohol
 Condensation
 -Si-OH + -Si-OR --Si-O-Si- + ROH (7)
 SAlcoholysis


 Gelation. After condensation reactions begin, gelation can occur (see Figure 2-6

for structure changes). Gelation is considered the growth period for the colloidal

particles, which grow by polymerization or aggregation, depending on the conditions.

Eventually, as the last links are made, the sol's viscosity increases to the point where the

sol becomes a semi-rigid gel that consists of a network of pores filled with liquid.

 Aging. Aging occurs after gelation. Here, terminal silanols re-orient themselves

and react with each other to form additional network linkages. This results in shrinkage

of the gel and forces the removal of some liquid from the pores. As the gel stiffens,

"Ostwald ripening" occurs. This is the phenomenon whereby necks begin to form

between primary particles causing the filling of small pores (see Figure 2-6 part c).