Liquid Chromatography - The Preparation of Spherical Silica Gel
The Preparation of Spherical Silica Gel
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 has also been shown possible to disperse a silica sol in the form of an emulsion in a suitable organic solvent where the droplets gel in spherical form (12). Unfortunately, details for the preparation of spherical silica have tended to be kept very confidential for commercial reasons and so information is a little sparse. One of the first methods reported was that of Le Page et al (13,14). A stable silica sol. (generated at low pH so that gelling does not take place) is passed through a non aqueous solvent in such a manner as to produce droplets. These droplets rapidly solidify and are then filtered off, dried, and heated to 400˚C to 800˚C to form a rigid xerogel. The structure of the resulting silica is strongly affected by the alkali metal content and the calcining temperature. The higher the temperature employed the lower the surface area and pore volume of the resulting silica.
An alternative method devised and patented by Unger (15) called the polyethoxy silane procedure involved 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 silica hydrogel. The hydrogel spheres are then washed and converted to the xerogel by heating. Spherical silica gel is readily available commercially, both in the form of silica and as different types of bonded phase. Today the majority of silica-based LC column packings are spherical although with modern methods of grinding, the so called irregular particles are more rounded and the difference between the performance of spherical and irregular particles is less clearly defined.