Fj. Arriagada et K. Osseo-asare, Controlled hydrolysis of tetraethoxysilane in a nonionic water-in-oil microemulsion: a statistical model of silica nucleation, COLL SURF A, 154(3), 1999, pp. 311-326
Citations number
37
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS
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.