The Mechanism of Chromatographic Retention - Retention and Exclusion > The Preparation of Silica Gel > Page 61
As the primary particles of silica gel, formed during the gelling process, can vary from a few Angstrom to several hundreds of Angstrom in diameter, the pores of the silica gel, which are formed by the interstices that exist between the primary particles, will have commensurate dimensions. Now, this range of pore diameter covers the molecular size of most of the solutes separated on LC columns. It follows, that silica gel will act as an exclusion media and be capable of separating substances on the basis of molecular size. The smaller molecules will enter the majority of the pores and thus, be retained the most. The larger molecules may be excluded from all the pores and, consequently, will be retained the least.
Contemporary silica gel particles that are used per se or for the production of bonded phases are spherical in shape and made differently. However, the chemical process of gel formation is very similar and leads to the same porous material of high surface area with very similar physical properties. Spherical particles of silica can be prepared by spraying a neutralized silicate solution (the colloidal silica sol) into fine droplets before gelling has taken place and subsequently drying the droplets in a stream of hot air. It is also possible to disperse a silica sol in the form of an emulsion in a suitable organic solvent where, by the addition of a suitable catalyst, the droplets can be made to gel in spherical form. Another method called the polyethoxy silane procedure involves a two stage process. Firstly tetraethoxy-silane is partially hydrolyzed to polyethoxysiloxane, a viscous liquid which is then emulsified in an ethanol water mixture by vigorous stirring. The stirring produces spheres of polyethoxysiloxane which by hydrolytic condensation, initiated by a catalyst, are changed to a silica hydrogel. The hydrogel spheres so formed are then washed and converted to the xerogel by heating. It is claimed that spherical particles are easier to pack and produce higher efficiencies although the difference if it occurs, is marginal. Nevertheless, spherical silica is perceived as 'state of the art packing material'.