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Sol-gel is a wet-chemical route in which a sol (a colloidal suspension of small particles) is converted into a gel (a continuous three-dimensional network) by hydrolysis and condensation of precursors such as metal alkoxides. The gel is then dried into a coating, film, fibre, powder or monolith. It is a processing method, not a single material, and the way the gel is dried decides whether you end with a dense xerogel or a highly porous aerogel.
Definition and scope: what “sol to gel” means
Sol-gel (also written “sol gel”, “sol to gel” or “sol and gel”) describes a sequence rather than a substance. A review of hybrid sol-gel coatings describes it as a versatile method used to produce a wide diversity of materials, from glasses to organic-inorganic hybrids, and summarises the process as formation of a colloidal suspension (the sol) followed by polymerisation into a three-dimensional structure (the gel) (PMC7182864, read 2026-11-08).
Because “sol” and “gel” name two different physical states, the phrases “sol or gel” and “gel and sol” are not interchangeable: the sol is the starting dispersion, the gel is the network that forms from it, and the transition between them is the point of the method. See also what is nanotechnology for how bottom-up routes like this fit into nanomaterial synthesis.
How the sol-to-gel transition works
Step 1: hydrolysis
For metal-alkoxide precursors, the first step is hydrolysis in the presence of water, which produces hydroxyl groups on the metal centre (PMC7182864).
Step 2: condensation and gelation
Condensation then links the hydrolysed units into a three-dimensional network; for silicon this is the Si-O-Si network. The sodium-silicate aerogel study likewise describes a two-step reaction, hydrolysis followed by condensation, and notes that gelation time is inversely proportional to the rate of gel formation (PMC8698247, read 2026-11-08). Silicon alkoxides such as TEOS and TMOS react more slowly than titanium, aluminium or zirconium alkoxides, which gives better control (PMC7182864).
Step 3: ageing and drying
After gelation, syneresis and ageing strengthen the network and expel pore liquid; ageing is defined as the change in structure and properties after gelation (PMC8698247). Gelation by solvent evaporation is how coatings, films and fibres are produced, and casting and dip coating are established variants (PMC7182864).
Sol vs gel vs xerogel vs aerogel
The two papers support the following distinctions. Every row is taken from the cited sources.
| Form | What it is | What the sources state |
|---|---|---|
| Sol | Colloidal suspension | The first stage of the process (PMC7182864) |
| Gel | Three-dimensional network formed by condensation | Strengthened by syneresis and ageing after gelation (PMC8698247) |
| Xerogel | Gel dried by normal evaporation | Network contracts; volume is usually 5 to 10 times less than the wet gel (PMC8698247; PMC7182864 gives the same range) |
| Aerogel | Gel dried without capillary pressure (for example supercritical drying) | Shrinkage is minimal; usually about 99% porous (PMC8698247, PMC7182864) |
The decisive variable is drying. The key factor determining shrinkage is the capillary pressure of the pore liquid, and lower-surface-tension solvents give lower-density products (PMC8698247).
What controls the outcome: a variable checklist
- Catalyst: hydrolysis and condensation may be catalysed by acids or bases. Per the review, acid catalysis gives highly branched polymer, while base catalysis gives colloidal particles with linear chains or a ringed shape (PMC7182864).
- Other reaction variables: pH, temperature, solvent, the alkyl group of the precursor and the water-to-alkoxide ratio all affect the result (PMC7182864).
- Precursor: silicon alkoxides react more slowly and are easier to control than Ti, Al or Zr alkoxides (PMC7182864).
- Ageing and washing: in the sodium-silicate route specifically, Na+ left in the gel causes brittleness and cracking and has to be washed out (PMC8698247). This applies to that route only, not to alkoxide routes in general.
- Drying route: evaporative drying shrinks the network; supercritical drying avoids it but affects cost efficiency, continuity and safety of the process because of the high temperature and pressure involved (PMC8698247).
Where it matters for labs
The sources list the practical attractions as simple, inexpensive equipment, low-temperature synthesis and control of stoichiometry (PMC7182864). The recurring limitation is shrinkage control, which the review calls challenging; incorporating organic components is one way to obtain thick, crack-free films. For a lab, shrinkage and cracking during drying are therefore the failure modes to watch, and they point back to the drying route, ageing and residual-ion variables above. One reported ambient-pressure TEOS route dried at room temperature for 24 h and then at 323 K for 24 h, giving a BET surface area of 869 m2/g (PMC8698247, citing Brinker); this is a literature result, not a protocol. For related fibre processing see electrospinning, and for a polymerisation initiator commonly handled in the same labs see ammonium persulfate.
Common misunderstandings
- Sol-gel is a material. It is a route; the product can be a glass, a hybrid coating, a xerogel or an aerogel.
- A gel is just a thick sol. A gel is a connected network formed by condensation, not a more concentrated dispersion.
- Xerogel and aerogel are the same. They start from the same kind of gel; the drying route separates them (5 to 10 times volume loss versus minimal shrinkage).
- Conditions from one precursor transfer to another. Reactivity differs between silicon and Ti, Al or Zr alkoxides, and pH windows reported for sodium silicate should not be applied to alkoxides.
FAQ
What is sol-gel in one sentence?
A route that turns a colloidal suspension (sol) into a three-dimensional network (gel) through hydrolysis and condensation, then dries it into a film, powder, fibre or monolith.
What is the difference between a sol and a gel?
A sol is a colloidal suspension of particles; a gel is the connected three-dimensional network that forms from it after condensation.
How does a sol turn into a gel?
Hydrolysis of the precursor produces reactive hydroxyl groups, and condensation joins them into a network (Si-O-Si for silicon). The rate of this reaction sets the gelation time.
Why do xerogels shrink and aerogels do not?
Capillary pressure of the pore liquid collapses the network during evaporative drying; drying that avoids capillary pressure, such as supercritical drying, leaves a highly porous aerogel.
Sources
- “Hybrid Sol-gel Coatings for Corrosion Mitigation: A Critical Review”, PMC7182864, pmc.ncbi.nlm.nih.gov/articles/PMC7182864, read 2026-11-08.
- “Sodium silicate-derived aerogels: effect of processing parameters”, PMC8698247, pmc.ncbi.nlm.nih.gov/articles/PMC8698247, read 2026-11-08.
