Synthesis of nanometer-sized silica by controlled hydrolysis in reverse micellar systems

The synthesis of nanometer-sized silica particles by the base-catalyzed, controlled hydrolysis of tetraethoxysilane (TEOS) in a nonionic reverse micellar system is described. Spectrofluorometric techniques were used to characterize the reverse micellar solutions. Particle characterization was conducted by transmission electron microscopy. The effect of the water-to-surfactant molar ratio (R) on particle size and size distribution was investigated over a wide range of R values (0.50 to 3.54). Stable dispersions of amorphous silica with mean particle diameters in the range of 46 to 68 nm were produced. Small (46 nm) and extremely monodisperse particles (polydispersity below 4%) were obtained at intermediate R values (1.4), whereas both particle size and polydispersity increased at lower and higher R values. The effects of R on particle size and size distribution are discussed in terms of water “reactivity” (i.e., proportion of bound to free water), concentration of reverse surfactant aggregates, distribution of hydrolyzed TEOS molecules among aggregates, and dynamics of intermicellar matter exchange. A mechanistic model for particle nucleation and growth in these systems is proposed.