Nanostructured organic-inorganic hybrid materials: Kinetic control of the texture

The organic-inorganic hybrid materials obtained by the sol-gel route constitute a new class of solids prepared directly from a molecular precursor in mild conditions. These materials are better described as kinetically controlled solids instead of thermodynamically unstable materials. The reproducibility is observed only when the experimental conditions and the purity of chemicals are controlled with the precision used in analytical measurements. The results presented here illustrate the importance of all the parameters involved during the hydrolytic polycondensation of silicon alkoxides. They clearly evidence the narrow connections between the macroscopic data which describe the texture of the solids (specific surface area, porosity) and the parameters which control the kinetics of all the reactions involved in the process: the nature of the solvent, nature and concentration of catalyst and reagents, and temperature. However, some trends could be observed: polycondensation at silicon is also highly dependent on the geometry of the molecular precursor, "rigid" or "flexible". Moreover, very interestingly the aging conditions (time and temperature) exhibit a drastic influence on the textural properties of the material whereas the level of condensation at silicon is not influenced. These results clearly show that polycondensation at silicon and the texture are not controlled in the same steps of the overall process.