From (Sub)colloidal Growth to the Gel Structure

An important consideration in the study of the sol–gel method is that there is no universal recipe for the preparation of a selected catalytic formulation. This means that it is not possible to provide a scheme for the preparation of a catalyst using the sol–gel process, as the synthesis route depends on the type of the selected approach. Furthermore, since its discovery, the sol–gel method has undergone continuous development and new synthesis routes have been designed. Generally speaking, we can consider the following as starting materials: alkoxides and organic molecules (chelating agents, surfactants, esters, organic acids), inorganic salts (nitrates, chlorides), solvent—usually alcohol (methanol, ethanol)-water. Catalysts when it is necessary to increase the speed of hydrolysis or simply to change the particle growth mechanism. The structure of the gel, responsible for the properties of the final material, is closely related to the hydrolysis and condensation pathway and the cluster growth mechanism. In ‘colloidal' gels, the network consists of the agglomeration of dense colloidal particles, whereas in ‘polymeric' gels, the particles have a polymeric substructure resulting from the aggregation of sub-colloidal chemical units. The latter are an entanglement of randomly branched polymer chains. Actually, it is not possible to predict which type of cluster will form in a specific case, due to the complexity of the system. However, if a particular synthesis protocol is followed exactly, the same cluster can be reliably synthesised in most cases, very often with high yields and in large quantities. This is one of the great advantages of this methodology. Among the factors influencing hydrolysis and condensation reactions, water is a key parameter regulating the sol-to-gel transition and gelation time. In this chapter, we will see how it also affects the textural and structural properties of the synthesised material.