A sol is the dispersion of very small-scale (2nm to 1µm) solid particulates in a liquid. This is one particular type of colloidal system, a general classification of these systems is given in Table 1.
Table 1 Classification of colloidal systems
| System | Dispersed Phase | Dispersion Medium |
|---|
| Sol |
Solid |
Liquid |
| Emulsion |
Liquid |
Liquid |
| Solid Emulsion |
Liquid |
Solid |
| Foam |
Gas |
Liquid |
| Fog, Mist or Aerosol |
Liquid |
Gas |
| Smoke |
Solid |
Gas |
| Alloy |
Solid |
Solid |
There are two generic variations of the sol-gel technique, the colloidal route and the polymeric (or alkoxide) route.
For the colloidal sol-gel route, the sol is created by dispersing fine particles in a liquid which is often water.
For the polymeric sol-gel route, there is a pre-stage before the sol is formed. The precursor (often a chemical called an alkoxide) is dissolved in a solvent, usually an alcohol. This produces a true solution i.e. a single-phase liquid. This solution is then treated, or activated to produce polymeric chains in the alcohol. This is the sol, or more accurately, a polymeric sol.
Sols are often stable and do not change significantly with time, however, when they are destabilised (or activated) they can solidify to form a gel.
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