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 siz e 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 silic a particles with the number of surfactant aggregates indicated that, depend ing on the water-to-surfactant molar ratio (R), one particle was produced o ut of similar to 10(4) to 10(6) surfactant aggregates. The minimum number ( i(c)) of hydrolyzed TEOS monomers required to form a stable nucleus was est imated to be equal to two. The nucleation efficiency factor (F), i.e. the p robability of effecting nucleation in a reverse micelle that contains enoug h monomers to produce a stable nucleus, was found to increase with R for re latively low ammonia concentrations ([NH3] = 1.6 wt.%). A decrease in the: apparent nucleation efficiency factor (F) was observed at high R values wit h more concentrated ammonia and this was attributed to silica nuclei aggreg ation promoted by enhanced intermicellar collisions and intermicellar excha nge. (C) 1999 Elsevier Science B.V. All rights reserved.