TWI Frequently asked questions
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 .
