JMS Letters
Dear Sir, Electrospray Ionization in the Study of Polycondensation of Si(OEt) 4 β
The sol-gel process for the preparation of glasses and ceramics is based on hydroxylation and condensation reactions of suitable compounds. The use of the sol-gel method has attracted great scientific interest in recent years for making advanced materials and for designing devices with very specific properties.
The most versatile precursors for the sol-gel synthesis of oxides are alkoxides, but organic and inorganic salts are often used.
One unique feature of the sol-gel process is the ability to go all the way from the molecular precursor to the product, thus allowing better control of the whole process and the synthesis of 'tailormade' materials. This advantage, together with the mild conditions of synthesis ('soft chemistry'), makes the sol-gel method particularly suitable for yielding products with good control over composition, texture and microstructure. 1 By varying the chemical conditions under which alkoxides are polymerized, very different structures can be formed, which range from randomly branched polymers to colloidal particles.
Structural evolution during the sol to gel and gel to solid transitions needs to be fully understood to achieve real mastery of the sol-gel process. The properties of a gel and its response to heat treatment are very sensitive to the structure already formed during the sol stage.
The reactivity of alkoxides towards nucleophilic reagents, especially water, is the most outstanding chemical property of direct interest for sol-gel processing. 2 By hydrolysis and condensation reactions, such molecular compounds are transformed into highly cross-linked oxo-based macromolecular networks. Since hydrolysis is the main reaction in the transformation of alkoxide precursors to oxides, its importance justifies the great interest devoted to understanding the role of hydrolysis reaction mechanisms and kinetics in the nature, structure and morphology of the final solids.
Owing to the low reactivity of tetraalkoxysilanes, Si(OR) 4 , in the past and even recently most work has been done understanding the hydrolysis of silicon alkoxides, particularly silicon tetramethoxide and tetraethoxide. 3 Silica gels are common in nature, examples being opals and agates, and the hydrolysis and condensation of silicates is not new. More recently it has been demonstrated that by control of the process parameters, the H 2 O/Si ratio and the nature and concentration of the catalyst, it is possible to vary the structure and properties of sol-gel silicates over wide ranges. Even in the case of alkoxysilanes, however, only the mechanisms of the first steps of the sol-gel process are fairly well known. It is generally argued that hydrolysis proceeds by bimolecular nucleophilic displacement reactions involving pentacoordinate intermediates or transition states. 4,5 By analogy with carbon chemistry, siliconium ions Si(OR) 3
C have also been suggested as possible intermediates. 6 Mass spectrometry has been successfully employed to study the reaction intermediates formed either in the gas phase, by chemical ionization experiments, 7 or in the condensed phase, by continuous-flow fast atom bombardment. 8 In the former case the five-coordinate [Si(OEt) 4 OH] C species was found to be the most reactive initiator of -Si(OEt) 2 -chain formation, whereas in the