Silica microparticles precipitation by two processes using supercritical fluids

Silica microspheres are of great interest for several applications in areas such as medicine, biochemistry, colloidal chemistry and aerosol research. In this work, the sol–gel method was used to obtain such microparticles in two different ways. One of the methods applied was supercritical reaction; a high temperature process using ethanol as a supercritical fluid (SCF reaction). Another approach to obtaining silica microparticles was Supercritical AntiSolvent (SAS) process, as a low temperature process, using supercritical carbon dioxide (scCO2). A tetraethyl orthosilicate (TEOS) and water solution (1:4 molar ratio) was dissolved in ethanol and then pumped into an autoclave at 100 bar and 523 K for the SCF reaction process, and at 120 bar and 313 K in the case of the SAS process. All the experiments led to a successful precipitation of silica microparticles, in the micrometer range. In all cases spherical morphology and no agglomeration was found. Furthermore, the main textural characteristics of such powders were obtained by nitrogen physisorption experiments. Results reveal spherical microparticles with nanopores smaller than 1 nm in size when using supercritical ethanol, and bulky microparticles with smooth surfaces when using scCO2.