Preparation of SiO₂ nanoparticles in a non-ionic reverse micellar system

The preparation of nanosize, monodisperse silica particles by controlled hydrolysis of tetraethoxysilane (TEOS) in a non-ionic surfactant/ammonium hydroxide/cyclohexane reverse micellar system is described. Particles in the range of 50 to 70 nm are produced with standard deviations around the mean diameter below 8.5%. As the water to surfactant molar ratio increases, the particle size decreases and the size distribution becomes narrower. Spectrofluorometric techniques are used to evaluate the surfactant aggregation numbers and the nature (i.e., free or bound) of water solubilized in the aggregates. The effects of the water content on the particle size and size distribution are discussed in terms of the concentration and structure of the aggregates, as well as in terms of the relative proportion of bound (i.e., bound to the oxyethylene groups of the surfactant molecule) and free water. Possible mechanisms leading to the formation of the particles in this system are discussed.