  In the synthesis of metallic nanoparticles in microemulsions, we hypothesized
that particle size is mainly controlled by the reaction rate. Thus, the changes
observed on the particle sizes as reaction conditions, such as concentrations,
temperature, type of surfactant used, etc., are varied should not be correlated
directly to the modification of those conditions but indirectly to the changes
they produce on the reaction rates. By means of time resolved UV-vis
spectroscopy, we measured the reaction rates in the production of Pd
nanoparticles inside microemulsions at different reactant concentrations,
keeping all the other parameters constant. The measured reaction rates were
then correlated with the particle sizes measured by transmission electron
microscopy (TEM). We found that nanoparticle size increases linearly as the
reaction rates increases, independently of the actual reactant concentrations.
We proposed that the kinetics is controlled mainly by the diffusion of the
reducing agent through the surfactant monolayer covering the microemulsion
membrane. With this model, we predicted that particle size should depend
indirectly, via the reaction kinetics, on the micelle radius (v0 ~ r^-3), the
water volume (v0~vw^3) and the total microemulsion volume (v0~vT^-3), and
temperature (Arrhenius). Some of these predictions were explored in this
article.
