Direct preparation of silica hollow spheres in a water in oil emulsion system: The effect of pH and viscosity

Silica hollow particles have been prepared in this work in using a water in oil (W/O) emulsion system at room temperature that employs a water-soluble amine as catalyst and tetraethylorthosilicate (TEOS) as the silica source. The pH value of the aqueous phase and the viscosity of external oil phase were found to be the key factors contributing to the formation of stable and regular spherical silica hollow particles. When the pH value of the aqueous phase was controlled between 8 and 9, silica hollow particles could be synthesized. The shell thickness of the hollow particles increased with the length of the hydrocarbon tail of the amine catalyst. The viscosity of the external oil phase determined the shape regularity of the spherical silica hollow particles. If viscous silicon oil was used as the external oil phase instead of kerosene, perfect silica hollow spheres could be fabricated. The kinetics of the formation of silica hollow particles was believed to be based on the difference between the hydrolysis rate and the condensation rate of TEOS, which can be adjusted by the pH value of the aqueous phase and the viscosity of external oil phase.