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1.2. Fundamental Sol-Gel Process
The overall sol-gel process, as the name implies, usually involves two stages:
precursors initially form high molecular weight but still soluble poly-intermediates, a sol,
and the intermediates further link together to form a three-dimensional network, a gel. 15
The precursors for a sol-gel reaction could be either inorganic salts or organic compounds,
known as metal alkoxides. The formation of a gel from both types of precursors will be
illustrated in the following with silica as the primary example, since most of our
understanding of the sol-gel process is derived from silica composition.
1.2.1. Inorganic Salt Precursors
Porous silica gels can be prepared with inorganic salts, such as sodium silicates or
potassium silicates. 16 In this method, two types of chemical reactions are involved:
soluble silicates hydrolyze to monomeric Si(OH)4 under acidic conditions, reaction 1-1,
followed by polymeric condensation of the silicic acid, reaction 1-2.
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(1-1)
(1-2)