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What is a sol (in the context of sol-gel technology)?

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

SystemDispersed PhaseDispersion 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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