Synthesis of Nanoparticles in Microemulsions

Abstract

The technique of chemical reactions in microemulsions to produce nanoparticles has already 20 years of history behind, but the mechanisms to control the final size and the size distribution are still not well known. The knowledge of the mechanism is a crucial step in order to extend the potential applications of this technique. Nowadays there is a great interest in nanotechnologies and the developing of simple and reproducible methods to synthesize nanomaterials has attracted the interest of many researchers. The microemulsion method is a good candidate for this purpose. Microemulsions are thermodynamically stable systems composed of two inmiscible liquids (usually, water and oil) and a surfactant. Droplets of water-in-oil (W/O) or oil-in-water (O/W) are stabilized by surfactants when small amounts of water or oil are used, respectively. The size of the droplets can be controlled very precisely just by changing the ratio R = [water or oil] / [surfactant] in the nanometer range. These nanodroplets can be used as nanoreactors to carry out chemical reactions. It was initially assumed that these nanodroplets could be used as templates to control the final size of the particles, however, the research carried out in the last years has shown that besides the droplet size, several other parameters play an important role in the final size distribution. The purpose of this review is not to summarize all the results obtained in microemulsions, for which other reviews are available, but to provide a general picture of the different mechanisms involved. The influence of different parameters, such as exchange rate constant, film flexibility, reactant concentration, etc, on the final particle size will be discussed on the basis of Monte Carlo simulation results. Although this study is more focussed on reactions carried out in W/O microemulsions the general conclusions could be also applied to O/W microemulsions.