SAXS STUDY OF GELATION AND PRECIPITATION IN TITANIUM-BASED SYSTEMS

The chemical reactivity of transition metal alkoxides must be controlled in order to obtain sols and gels. In TiO2-based systems, this control can be achieved through acid catalysts. H+ ions catalyze the hydrolysis of Ti(OR)4 into TiOH through the protonation of the OR groups; however the protonation of TiOH inhibits the nucleophilic attack of Ti by TiOH, hence condensation reactions are hindered. A continuous range of final Products including stable sols, transparent gels, turbid gels and precipitates can be synthesized by varying the ratio H+/Ti used for the hydrolysis reaction. The structures of these materials have been investigated through small angle X-ray scattering. The polymers grow as fractal objects; remarkably, the fractal dimension, d(f), varies continuously according to the ratio H+/Ti. Sols have d(f) almost-equal-to 1.5-2; gels range between d(f) almost-equal-to 2 (transparent) and d(f) almost-equal-to 2.9 (turbid); precipitates correspond to d(f) almost-equal-to 3.