Fj. Arriagada et K. Osseo-asare, Synthesis of nanosize silica in a nonionic water-in-oil microemulsion: Effects of the water/surfactant molar ratio and ammonia concentration, J COLL I SC, 211(2), 1999, pp. 210-220
The effect of ammonia concentration on the region of existence of single-ph
ase water-in-oh microemulsions has been investigated for the system polyoxy
ethylene (5) nonylphenyl ether (NP-5)/cyclohexane/ammonium hydroxide. The p
resence of ammonia decreases the size of the microemulsion region. A minimu
m concentration of surfactant (estimated at about 1.1 wt%) is required for
solubilization of the aqueous phase; this value is not significantly affect
ed by ammonia concentration. As indicated by fluorescence spectral data, th
e transition between bound and free water occurs when the water-to-surfacta
nt molar ratio is about I and the presence of ammonium hydroxide does not a
ppear to have a significant effect on this. Ultrafine (30-70 nm diameter),
monodisperse silica particles produced by hydrolysis of tetraethoxysilane (
TEOS) in the microemulsion show a complex dependence of the particle size o
n the water-to-surfactant molar ratio (R) and on the concentration of ammon
ium hydroxide. At relatively low ammonia concentration in the aqueous pseud
ophase (1.6 wt% NH3) the particle size decreases monotonically with increas
e in R. However, for higher ammonia concentrations (6.3-29.6 wt% NH3) a min
imum in particle size occurs as R is increased. These trends are rationaliz
ed in terms of(a) the effects of the concentration, structure, and dynamics
of the NP-5 reverse micelles on the hydrolysis and condensation reactions
of TEOS, and (b) the effects of ammonia concentration on the stability of t
he microemulsion phase, the hydrolysis/condensation reactions of TEOS, and
the depolymerization of siloxane bonds. (C) 1999 Academic Press.