Controlled hydrolysis of tetraethoxysilane in a nonionic water-in-oil microemulsion: A statistical model of silica nucleation

Nanosize silica particles were synthesized by the base-catalyzed hydrolysis of tetraethoxysilane (TEOS) in the polyoxyethylene (5) nonylphenyl ether (NP-5)/cyclohexane/NH₄OH water-in-oil microemulsion system. The particle size data obtained by transmission electron microscopy were analyzed in terms of a statistical nucleation model based on the reverse micellar populations, the partition of TEOS molecules between the reverse micellar pseudophase and the bulk oil phase, and the Poisson distribution of TEOS molecules and hydroxyl ions among the reverse micelles. Comparison of the number of silica particles with the number of surfactant aggregates indicated that, depending on the water-to-surfactant molar ratio (R), one particle was produced out of ~ 10⁴ to 10⁶ surfactant aggregates. The minimum number (i(c)) of hydrolyzed TEOS monomers required to form a stable nucleus was estimated to be equal to two. The nucleation efficiency factor (F), i.e. the probability of effecting nucleation in a reverse micelle that contains enough monomers to produce a stable nucleus, was found to increase with R for relatively low ammonia concentrations ([NH₃] = 1.6 wt.%). A decrease in the apparent nucleation efficiency factor (F) was observed at high R values with more concentrated ammonia and this was attributed to silica nuclei aggregation promoted by enhanced intermicellar collisions and intermicellar exchange.