2. The different gel families

Gels are the result of so-called "soft chemistry" reactions, which take place in the liquid phase in the presence of an ionic or covalent precursor. The latter reacts in the presence of a reagent, catalysts or under the effect of temperature to form a solid colloidal or macromolecular species which is then organized into a network. The aim of this section is to describe the different ways in which these species can be synthesized, and to give examples of operating conditions for obtaining gels for supercritical drying.