SOL-GEL SYSTEMS WITH CONTROLLED STRUCTURE FORMED IN SURFACTANT MEDIA

The structure and mechanisms of formation of silica and zirconia gels produced by alkoxide hydrolysis [tetramethylorthosilicate (TMOS) and Z r (OPri)(4))] in different surfactant media have been investigated usi ng small-angle neutron scattering and other techniques. Inverse micell e and lamellar phase systems have been formed with non-ionic octylphen yl polyether alcohol surfactants, C-8 Phi E(x), where x, is 5 and 10, respectively. In these three-phase systems (water-surfactant-organic s olvent) a large proportion of water is bound to the hydrophilic polyox yethylene chains of the surfactant molecules. This has an important ro le in controlling the formation of the gel and its structure. Inverse micelles act as nucleation sites for the formation of very small oxide particles (less than or equal to 3 nm), which subsequently aggregate to give a fractal structure (D approximate to 2.3). The lamellar phase s exist as microdomains (0.1-1 mu m), and alkoxide hydrolysis occurs p redominantly in the surrounding zones, to give fractal aggregates (D a pproximate to 1.9) corresponding to diffusion-limited cluster aggregat ion (DLCA). Under shear, such lamellar phases are aligned and then pro duce oriented silica gels with an anisotropic structure. Cylindrical m icelles can be formed with the cationic n-alkyl trimethyl ammonium sur factant. With these systems hydrolysis of TMOS occurs in the aqueous p hase surrounding the micelles. Such relations can induce the organisat ion of the micelles to a hexagonal phase. Subsequent elimination of su rfactant results in silica xerogels containing cylindrical pores of co ntrolled size.