The Mechanism of Chromatographic Retention - Retention and Exclusion > The Preparation of Silica Gel > Page 60

Initially, silicic acid is released,

Na2SiO3 +H2O + 2HCl = Si(OH)4 + 2NaCl

However, the free acid quickly starts to condense with itself with the elimination of water to form dimers, trimers and eventually polymeric silicic acid.

The Polymerization of Silicic Acid

The polymer continues to grow, initially forming polymer aggregates and then polymer spheres, a few Angstrom in diameter. These polymeric spheres are called primary silica particles. The primary particles continue to grow until, at a particular size, the surface silanol groups (hydroxyl groups attached to the surface silicon atoms) on adjacent primary polymer particles, start condense. This condensation causes the primary particles to adhere to one another and at this stage the solution begins to gel. During this process, the primary particles of silica gel that are formed will have a range of diameters from a few Angstrom to many thousands of Angstrom depending on the conditions of formation. The size of the primary particles can be quite difficult to control and will depend, among other factors, on the conditions of synthesis, for example, reaction temperature, the pH of the reaction mixture at the time of gelling and even the subsequent treatment of the gel, including the conditions of washing.