Synthesis of nanosize silica in a nonionic water-in-oil microemulsion: Effects of the water/surfactant molar ratio and ammonia concentration

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.