The sol-gel synthesis of metal oxides can be performed via the hydrolysis and condensation of metal cations in aqueous solutions. This inorganic route provides a cheap and reliable way for industrial uses. However, many hydrolyzed precursors can be present simultaneously in the precursor solution and very few data are available on the formation of condensed species. The so-called Partial Charge Model provides a useful guide to describe and predict hydrolysis and condensation reactions in aqueous solutions. A charge-pH diagram can be established. It shows how condensation can be initiated via acid-base or redox reactions. The two basic condensation processes, olation and oxolation, are then discussed in terms of a charge-electronegativity diagram. They can lead either to small solute condensed species (polyanions or polycations) or to the formation of an infinite network (colloids, gels, or precipitates). Complexation by anionic species is finally discussed in the frame of an electronegativity-pH diagram. It also plays an important role during the formation of solid phases.
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