Topics - Silica Gel
Silica gel, an oxide of the element silicon, is an amorphous, highly porous, partially hydrated form of silica. Silica occurs naturally but, as such, is not sufficiently hydrated for chromatographic use and so the silica gel used in chromatography is prepared synthetically. Crystalline silica is, in fact, the anhydride of silicic acid and thus silica gel is a polymeric form of silicic acid. Irregular silica gel is prepared by releasing silicic acid from sodium silicate with hydrochloric acid. The silicic acid rapidly dimerises with elimination of water and then with further elimination of water forms trimers and in this way eventually polymerizes to a ‘whitish’ translucent liquid. The polymer continues to grow forming polymer aggregate spheres a few Angstrom in diameter. When the spheres reach a particular size, the surface hydroxyl groups from different spheres condense forming a gel. The porosity and surface area of the subsequent silica gel is determined by the size of these spheres which will depend on the conditions of formation and on the manner in which the gel is subsequently washed. When the gel is first formed it is very soft and is allowed to dry in trays for a number of days. During this period, the gel shrinks and exudes salinated water and forms a firm, almost rigid gel called the hydrogel. The hydrogel is then well washed, and heated to 120oC for a few hours producing a hard, white amorphous mass called the xerogel. The xerogel is ground and sieved to the desired particle size producing the silica gel that can be used in chromatography. Alternative methods of synthesis, based on the same principle have been developed to produce spherical silica directly which only needs to be screened to the appropriate size for chromatographic use. Spherical silica is the most popular form of silica gel in contemporary liquid chromatography. Silica gel is one of the more important substances used in liquid chromatography: it is not only used as a polar stationary phase per se, but is also the basic material from which most of the bonded phases are prepared.