Regulation of the Charge and Hydrodynamic Diameter of Silica Nanoparticles in AOT Microemulsions

Abstract

Stable SiO2 organosols have been obtained by hydrolyzing tetraethyl orthosilicate in water-in-oil microemulsions of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT, AOT) in n-decane. Phase analysis light scattering and photon-correlation spectroscopy have been employed to study the electrophoretic mobility and hydrodynamic diameter of silica nanoparticles as depending on the concentrations of water (0–10 vol %), chloroform in n-decane–chloroform mixtures (0–40 vol %), and an oxyethylated surfactant (Tergitol NP-4, 0–0.125 M), as well as temperature (0–60°C). The electrophoretic mobility of the nanoparticles has been found to increase by three to four times upon the addition of water and chloroform. Nanoparticles with a hydrodynamic diameter of 20 nm are negatively charged in the absence of water, while the surface charge reversal takes place at water and chloroform concentrations of higher than 0.1 and 17 vol %, respectively.