Additive-free continuous synthesis of silica and ORMOSIL micro- and nanoparticles applying a microjet reactor

The continuous wet chemical preparation of micro- and nanoparticles is a major challenge for the large-scale production of functional colloids. Here we present a general synthetic strategy for sol–gel-based materials via an additive free homogenous approach avoiding emulsion-based systems. A variety of different silica and organically-modified silica (ORMOSIL) spherical particles were prepared applying a condensation of prehydrolyzed alcoholic solution of organotrialkoxysilanes in a microjet reactor. This method presents a unique wet chemical production method for nano- and microscale materials. Methyl-, ethyl-, propyl-, vinyl-, phenyl-, and mixed ORMOSIL particles in the range of 75 nm–2 µm were successfully synthesized without the addition of stabilizing surfactants. The method was also investigated for the continuous preparation of pure silica particles, and we succeeded to produce continuously up to 23 g particles per minute. The influence of different organic groups on the crosslinking of the siloxane network was systematically studied applying various spectroscopic and thermoanalytical methods. The degree of condensation of the obtained particles depends on the organic rests of the trialkoxysilanes, which was studied with 29Si CP-MAS NMR. We were able to show that phenyl silsesquioxanes show less condensation in the particles than smaller alkyl or vinyl groups. In addition, the silane concentration has a significant influence on the particle size. Generally, smaller particle diameters are obtained after decreasing the silane concentration. The described process delivers a fast and large scale wet chemical production of various silsesquioxane and silica particles without the use of additives and is therefore suited for a variety of potential applications where high purity of the particles is necessary.