SOL-GEL PROCESSING OF TITANIA USING REVERSE MICELLAR SYSTEMS AS REACTION MEDIA

Titanium-isopropoxide-derived gels were obtained in reverse micellar m edia prepared from a decane/TX35/H2O-H2O2 mixture. Polymerization of t itanium isopropoxide, which is solubilized in the organic medium, occu rs through hydrolysis and condensation reactions, the rates of which a re controlled by the frequency of intermicellar exchanges, and by the amount of hydrogen peroxide (or water) contained in the reverse micell es. Viscosity and light scattering measurements have shown that micell ar aggregates of small size solubilize eight hydrogen peroxide molecul es per surfactant molecule (TX35). The gelation kinetics of titanium d ioxide depend on three main parameters: the number of H2O2 and/or H2O molecules per amphiphile (n), the alkoxide hydrolysis rate (h), and th e alkoxide molality (m). At the same n, h and m values, the rate of ge l formation is greater in the presence of hydrogen peroxide than in th e presence of water. Furthermore, gels formed with hydrogen peroxide h ave a better mechanical strength, as confirmed by various techniques s uch as thermoanalysis, X-ray and electron diffraction, adsorption isot herms, electron microscopy, thermoporometry and small-angle X-ray scat tering. Mesoporous solids were obtained by means of thermal treatment; they possess a structure formed by interconnected rods of titania. Pr imary aggregates exhibit fractal behaviour in the gel phase, which ind icates that their formation occurs by a cluster-cluster aggregation, r eaction-limited process. On heat treatment of the gels, pores are form ed which appear to have a bottle-like shape with a diameter of approxi mately 4 nm.