Sol gel method

In sol gel process initially a stable colloidal solution called sol is formed. The sol is a liquid suspension of solid particles ranging in size from 1 nm to 1 micron. It can be obtained by hydrolysis and partial condensation of precursors such as an inorganic salt or a metal alkoxide.  The further condensation of sol particles into a three dimensional network produces a gel material. The gel is a diphasic material in which the solids encapsulate the solvent.  The molecular weight of the oxide species produced continuously increases. The materials are referred to aqua sol or aqua gels when water is used as a solvent and aquosol or alcogel when alcohol is used.

The encapsulated liquid can be removed from a gel by either evaporative drying or with supercritical drying /extraction. The resulting solid products are known as xerogel  and aerogel  respectively. When gels are dried by evaporation, the dried product is called xerogel. When the gels are dried by supercritical drying, the dried gel is called aerogels. The aerogel retains high porosity and has very high pore volume.

Fig. 1. General scheme of preparation by solgel method