The Mechanism of Chromatographic Retention - Retention and Exclusion > The Preparation of Silica Gel > Page 59

Retention and Exclusion

All stationary phases based on silica gel exhibit exclusion properties. Polystyrene stationary phases are broadly similar in physical form but their exclusion properties tend to be less significant than those of silica gel. The exclusion properties of silica gel arise from its method of formation and so the processes involved will help reveal the physical nature of the gel.

Silica gel is an amorphous, highly porous, partially hydrated form of silica made from the two most abundant elements in the earth's crust, silicon and oxygen. More than 55%w/w of the earth's surface consists of either silica (silicon dioxide) or silicates (silica combined with metal oxides). Silica found naturally is not significantly hydrated and, although it can exist in both crystalline and amorphous forms, it usually occurs as quartz, cristobalite or tridymite crystals. Quartz, when crushed by earth movement and weathered by air and water forms sand. Each silicon atom in both quartz and cristobalite is associated with four oxygen atoms but, in cristobalite the silicon atoms are thought to be oriented in a similar manner to the carbon atoms in diamond where the oxygen atoms would be considered to be situated half way between the 'carbon' atoms. In quartz, the silicon atoms adopt a helical configuration, and consequently, the crystals are 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).