Revising the synthesis of Stöber silica nanoparticles: A multivariate assessment study on the effects of reaction parameters on the particle size

The effects of parameters such as concentration of water, ammonia, ethanol and tetraethoxysilane (TEOS) on the size of silica nanoparticles synthesised according to the Stöber method have been evaluated according to a multivariate approach. A 24 factorial design was employed in order to evaluate the magnitude of the effects and their interactions on the average size of nanoparticles, aimed at explaining the controversial results described in the literature. Silica spheres ranging from 9 to 800 nm, depending on the experimental conditions, were obtained. Univariate and multivariate studies indicated that ammonia, water and TEOS showed significant effects on the size of the nanoparticles, which, on average, increases as the concentrations of these reagents increase. The Pareto chart indicates that all reagents present significant mean interaction effects at 95% of confidence. The strategy employed in the present work allowed to conclude that effect of each variable on particle size depends on the levels of other ones, that is, a given reagent can affect positively or negatively the particle size, depending on the synthesis conditions. Therefore, the effect of each variable on the particle size found in the present work as well as those already described in the literature must be taken with caution because their values depends on the concentration ranges under study.