h i g h l i g h t s The influence of experimental parameters on the particle size of SiO 2 was studied. A model was created by a multi-variate statistical analysis using a factorial design. This model predicts the particle size in function of experimental condi-tions. Only the molar amount of NH 3 , H 2 O and the TEOS to H 2 O molar are sig-nificant. The surface of nanoparticles was modified in situ using a silane cou-pling agent. g r a p h i c a l a b s t r a c t a b s t r a c t The synthesis of colloidal SiO 2 nanoparticles was achieved via the base-catalyzed hydrolysis and poly-condensation of tetraethyl orthosilicate (TEOS) in an alcoholic medium with the careful variation of the concentrations of water, ammonia and TEOS. In this study, we investigated the influence of experimental parameters on the average size of nanoparticles by means of a multivariate statistical analysis using a factorial design to create one model to predict particle size in function of experimental conditions. This model predicted that it is possible to synthesize silica nanoparticles with size in the range from 17 to 333 nm, according to the chosen condition for the amount of water, ammonia and TEOS. It was observed that only the molar amount of NH 3 , the molar amount of H 2 O and the effect from interaction between molar amount of TEOS and H 2 O were significant. Subsequently, surface modification of the colloidal silica was performed in situ using the coupling agent 3(trimethoxysilyl)propyl methacrylate (TMSPM).