Effect of mixing methods and chemical concentrations on the homogeneity of SiO2 microsphere via Stober and modified Stober methods

Different mixing processes and chemical compositions were performed using the Stöber and modified Stöber methods to improve the homogeneity of SiO2 microspheres. The direct mixing process with proper stirring speed (300 rpm) and duration (24 h) produced an excellent uniform size of 0.38 µm with a relative standard deviation (RSD) of 5.26% compared to the 0.38 µm with 18.41% RSD produced by the injection mixing process. In addition to the mixing process, the concentrations of tetraethyl orthosilicate (TEOS), H2O, and NH3 also affect the hydrolysis and condensation reaction rate. However, excessive concentrations of these chemicals can cause an imbalance in the reaction rates and aggregation process, leading to a multi-modal distribution. A mixture of 0.25 mol/l TEOS, 4.0 mol/l H2O, and 1.0 mol/l NH3 in ethanol produced highly uniform monodispersed SiO2 particles of 0.48 µm size with a small deviation of 6.25% RSD. It was found that K cations can modify the structure of the Stern layer, increasing the particle size from 0.49 to 1.15 µm but widening its distribution from 6.12% to 21.74% RSD, due to the fast diffusion and aggregation of the oligomer or primary particles. The bi-directional mixing method can increase the uniformity of SiO2 microspheres, reducing their RSD to 6.6% by creating a strong turbulence force through 30 cycles per hour of reversed mixing. This study demonstrates that the Stöber and modified Stöber methods can synthesize high-circularity SiO2 particles (between 0.93 and 0.97) with an improved particle size distribution.