41. Chattopadhyay, Pratibhash; Gupta, Ram B.  Supercritical CO2 based Formation of Silica Nanoparticles Using Water-in-Oil Microemulsions.    Ind. Eng. Chem. Res. 2003, 42, 465-472.


A novel method was proposed for the pptn. of silica nanoparticles using supercrit. CO2, in which supercrit. CO2 acts both as the antisolvent and as a reactant.  A water-in-oil microemulsion of an aq. sodium silicate soln. in n-heptane or isooctane was injected into supercrit. CO2 by a micronozzle, forming small droplets.  Supercrit. CO2 rapidly extd. the solvent from the droplets and reacted with the exposed surfactant-supported aq. sodium silicate reverse micelles, forming silica nanoparticles, sodium carbonate byproduct, and water.  The pptd. silica nanoparticles were washed with water and the surfactant, and the sodium carbonate-free particles were obtained by ultrafiltration.  Process parameters affecting silica nanoparticle size, such as water and surfactant concns., the type of solvent used in the microemulsion, and the CO2 pressure, were investigated.  Microstructure and size analyses of the pptd. silica nanoparticles were carried out using transmission electron microscopy and dynamic light scattering.  The proposed method was demonstrated by the pptn. of silica nanoparticles having sizes in the range of 20-800 nm.


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