A practical process for effective thickening of silica shells formed onto spherical cores by considering the variation in ionic strength during sol-gel reaction

A novel process for coating spherical silica cores with a thick, mesoporous shell to obtain core-shell type mesoporous silica particles (C-MSPs) was developed. The solution pH during the sol-gel reaction in the presence of cetyltrimethylammonium bromide (CTAB), which was used for micelle templates, was measured to model the concentrations of ionic species. Silicate ions were of particular interest because their concentration rapidly increased by the hydrolysis of tetraethyl orthosilicate (TEOS) at high concentrations. The silicate ions present in the reaction mixture were determined by rate equations for the hydrolysis of TEOS and the condensation of silica. Estimating the ionic strength based on the modeled concentrations of ionic species in the solution allowed us to design an appropriate process of coating particles with a mesoporous silica shell. Mesoporous shells were effectively thickened by the continuous addition of TEOS at controlled feeding rates, where the concentration of silicate ions was kept lower than the solubility of silica, which helped suppressing the generation of secondary particles (by-products). Micron-sized C-MSPs with a mesoporous shell around a submicron-sized silica core were successfully obtained by the co-addition of TEOS and CTAB. An approximately 400 nm thick mesoporous shell with a porosity similar to that of conventional thin shells was formed onto 460 nm spherical cores. The concept of controlling the ionic strength of the reaction solution can also be applied to other cores, which offers a practical approach to synthesize core-shell type mesoporous particles required in various future applications. • Core-shell mesoporous silica particles with a thick, uniform shell were synthesized. • The synthesis process was proposed based on the ionic strength in the reaction. • The continuous supply rate of TEOS was designed according to silica solubility. • The shells thickened in an efficient manner by adding TEOS and CTAB. • The synthesis process was applicable to silica cores with other diameters.