K. Osseo-asare et Fj. Arriagada, Growth kinetics of nanosize silica in a nonionic water-in-oil microemulsion: A reverse micellar pseudophase reaction model, J COLL I SC, 218(1), 1999, pp. 68-76
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.