The Sol to Gel Transition

Giving a clear and widely accepted definition in chemistry and physics is always a pretty difficult task, especially because the frontier of science is continuously moving. Time is a good friend for science because knowledge accumulates and allows getting a wider and deeper understanding of nature; however, it is also more complicate organizing what are sometimes only fragments of a big puzzle. Definitions have also a historical development and they are modified according to new knowledge that accumulates or simply with a change of perspective. This short book is dedicated to sol- to -gel transition in oxides, and therefore the first mandatory step is a clear understanding of what a gel and a sol are.

The definition of sol-gel process, clearly indicates that the formation of an inorganic chemical gel is the result of a “progressive” transformation; the process is continuous but different stages can be defined with the caution that clear identification of every stage is hampered by the overlapping of several competing processes such as hydrolysis and condensation.

The sol-to-gel transition is a critical phenomenon which is observed in systems of different nature and composition; several theories with different mathematical approaches have been developed with the main purpose of making a prediction of the gel point. These models have been applied to systems which grow in a stochastic way and to different practical problems. Most of these theories have been used for studying gelation in organic polymers and colloidal systems of particles but they also have been, even if a much smaller extent, applied to inorganic sol-gel systems. Some of these models reach a quite good correspondence with experimental results; in other cases, even if they fail to match with the data they are however an important tool for a basic understanding of the process. The models are also used to make previsions and to find answers to some fundamental questions: when the system will gel, which is the minimum number of reactions (bonds formation) to observe a sol to gel transition?

The sol to gel transition in inorganic systems is a process which is highly dependent on the synthesis conditions; the chemistry of silicon alkoxides, in particular, is very difficult to handle because so many different species can form from the very beginning of the process and a small change of the synthesis parameters is immediately reflected on the hydrolysis and condensation stages.

In the first chapter the definition of sol, gel, and sol-to-gel transition have been introduced; a main issue which still remains to be discussed is how to measure the gel point in the different systems. This is not a trivial aspect because only indirect measures are possible and no latent heat is released during the transition.

The sol-to-gel transition in a bulk gel is a macroscopic event, which can be followed using different analytical methods, even if an accurate determination of the gelation time is always a difficult task. It is possible to obtain reliable measures especially by a combination of techniques. The liquid phase of the gel remains, even after gelation, a peculiar chemical environment with a specific microviscosity which evolves during drying while diffusion inside the pores is always possible. This chapter is dedicated to a brief overview of the methods used to probe this microenviroment and sol-gel transition in thin films.