Supercritical CO2-based formation of silica nanoparticles using water-in-oil microemulsions

A novel method is proposed for the precipitation of silica nanoparticles using supercritical CO2, in which supercritical CO2 acts both as the antisolvent and as a reactant. A water-in-oil microemulsion of an aqueous sodium silicate solution in n-heptane or isooctane is injected into supercritical CO2 by means of a micronozzle, forming small droplets. Supercritical CO2 rapidly extracts the solvent from the droplets and reacts with the exposed surfactant-supported aqueous sodium silicate reverse micelles, forming silica nanoparticles sodium carbonate, byproduct and water. The precipitated silica nanoparticles are washed with water and the surfactant and sodium carbonate free particles are obtained using ultrafiltration. Process parameters affecting silica nanoparticle size such as water and surfactant concentrations, the type of solvent used in the microemulsion, and the CO2 pressure are investigated. Microstructure and size analyses of the precipitated silica nanoparticles are carried out using transmission electron microscopy and dynamic light scattering. The proposed method is demonstrated by the precipitation of silica nanoparticles having sizes in the range of 20-800 nm.