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bonded phases were also based on silica gel. More recently, polymeric stationary phases were introduced to provide materials that were insoluble in water and that were stable at extremes of pH. Silica Gel The Preparation of Irregular Silica Gel Silica gel is manufactured by releasing silicic acid from a strong solution of sodium silicate by hydrochloric acid. (Sodium silicate is prepared by heating sand at a high temperature in contact with caustic soda or sodium carbonate). Initially, silicic acid is released,                   Na2SiO3 +H2O + 2HCl = Si(OH)4 + 2NaCl and then the free acid quickly starts to condense with itself with the elimination of water to form dimers, trimers and eventually polymeric silicic acid. The polymer grows, initially forming polymer aggregates and then polymer spheres, a few Angstrom in diameter. These polymeric spheres are called primary silica particles. These primary particles continue to grow until, at a particular size, the surface

enantiomorphic and, thus, optically active. Silica, (SiO2) can be considered to be the anhydride of silicic acid, and in its naturally occurring crystalline form, does hydrate to silicic acid and, as a consequence, is very slightly soluble in water. However, the hydration is very slow and it is not practical to produce silica gel, for example, by directly hydrating quartz. The Preparation of Silica Gel Silica gel is manufactured by Patrick's process which consists essentially of releasing silicic acid from a strong solution of sodium silicate by hydrochloric acid. (The sodium silicate is usually manufactured by heating sand at a high temperature in contact with caustic soda or sodium carbonate

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

The Structure of Silica Gel The matrix of the primary silica gel particle consists of a core of silicon atoms joined together with oxygen atoms by siloxane bonds (silicon-oxygen-silicon bonds). On the surface of each primary particle some residual, uncondensed hydroxyl groups from the original polymeric silicic acid remain. These residual hydroxyl groups confer upon silica gel its polar properties. These hydroxyl groups react with the silane reagents to form bonded phases. The silica surface is quite complex and contains more than one type of hydroxyl group, strongly bound or 'chemically' adsorbed water and loosely bound or 'physically adsorbed' water. There are three types of hydroxyl group. The first is a single hydroxyl group attached to a silicon atom which has three siloxane bonds joining