Droplet-Mediated Size Control of Stober Silica Particles in Surfactant- and Organic Solvent-Free Emulsions

The aqueous Stöber synthesis is a promising organic solvent-free variant of the conventional Stöber synthesis. However, particle formation under aqueous, phase-separated conditions remains underexplored. Here, we develop a method for tuning the particle diameter from 80 to 350 nm by varying the stirring energy introduced into the two-phase system from 70 to 1050 mW/kg. TEOS droplet sizes, ranging from 50 to 800 μm and decreasing with an increase in stirring energy, were monitored to rationalize silica particle growth rates and final particle size. Particles and TEOS droplets were characterized in line by photon density wave spectroscopy and process microscopy and off line with light scattering techniques and electron microscopy. The final particle size distributions were highly sensitive to temperature. Below 20 °C, they were aggregation-dominated, whereas an increase in temperature reduced the final particle size. The lowest polydispersity was obtained at 25 °C. The results indicate that final particle size and growth rate decrease with an increase in TEOS droplet size. Direct control of particle size, high solid content, and low organic residue content in the final product were achieved. The resulting particles exhibited 6.6 ± 1.8 nm mesopores, in contrast to conventional dense Stöber particles, which are typically nonporous or predominantly microporous. A deeper understanding of the role of process parameters in this synthesis may enable greener and more sustainable routes to Stöber silica with an increased space time yield.