The cyclohexane-igepal inverse microemulsion, comprehensively established for the synthesis of silica nanoparticles
in an ammonia-catalyzed sol-gel process, was alternatively studied with an acid-catalyzed sol-gel process. Tetraethyl
orthosilicate (TEOS) was used as the silica precursor, while two different aqueous phases containing either HNO3 or
HCl at two different concentrations, 0.1 and 0.05 M, were examined in the presence and in the absence of NaF, a catalyst
of the condensation step. The evolution of the overall reacting system, specifically hydrolysis and polycondensation of
reaction intermediates, was monitored in situ by time-resolved small-angle X-ray scattering. No size variation of the
inverse micelles was detected throughout the sol-gel process. Conversely, the density of the micellar core increased after
a certain time interval, indicating the presence of the polycondensation product. The IR spectra of the reacting mixture
were in agreement with such a hypothesis. 1Hand 13CNMRmeasurements provided information on the soluble species,
the surfactant, and TEOS. The TEOS consumption was well fitted by means of an exponential decay, suggesting that
a first-order kinetics for TEOS transpires in the various systems examined, with rate constants dependent not only on the
acid concentration but also on its nature (anion specific effect), on the presence of NaF, and on the amount of water in
the core of the inverse micelle. The self-diffusion coefficients, determined by means of PGSTE NMR, proved that
a sizable amount of the byproduct ethanol was partitioned inside the inverse micelles. Characterization of the final
product was carried out by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and
transmission electron microscopy (TEM), which concurrently confirmed that the silica isolated from the inverse
nonionic microemulsion is not significantly different from the product of a bulk acid-catalyzed sol-gel synthesis. TEM
micrographs illustrated particles with diameters smaller than the diameter of the inverse micelles as determined by
SAXS, due to a shrinkage effect, in addition to nanostructured aggregates in the range 20-100 nm.
