Growth kinetics of nanosize silica in a nonionic water-in-oil microemulsion: A reverse micellar pseudophase reaction model

The growth kinetics of silica nanoparticles synthesized by the microemulsio n-mediated alkoxide hydrolysis method was investigated with tetraethoxysila ne (TEOS) as the silica precursor and polyoxyethylene (5) nonylphenylether (NP-5)/cyclohexane/ammonium hydroxide as the water-in-oil microemulsion sys tem. The time evolution of the mean diameter of the silica particles was de termined for different values of the water-to-surfactant molar ratio (R). P article growth was found to be a slow process, where under typical experime ntal conditions at room temperature ([TEOS] = 0.024 M, 29.6 wt% NH3, water- to-TEOS molar ratio (h) = 7.8) the particles achieved their terminal size a fter several days. During the early stages of the reaction, particle growth followed first-order kinetics, and the observed first-order growth rate co nstants decreased with increase in R. A reverse micellar pseudophase model (which considered the partition of reactants between the reverse micellar p seudophase and the bulk oil phase) was developed to analyze the growth kine tics under the hypothesis that TEOS hydrolysis was rate controlling. The ps eudophase model predicted an inverse relationship between the observed grow th rate and R, in agreement with experiment. The roles of steric effects an d the bound state of water molecules, in retarding the hydrolysis rate, wer e highlighted by examining the effect of R on the TEOS hydrolysis rate cons tant in the reverse micellar pseudophase. (C) 1999 Academic Press.