A novel process for precipitation of ultra-fine particles using sub-critical CO2

Supercritical CO2 has been utilized as solvent, cosolvent or antisolvent in several processes for production of ultra-fine solid particles with narrow size distribution. The key to the precipitation of such particles is to produce a very large, rapid and uniform supersaturation in the solution of a solid substance. This can be achieved either by a rapid and large reduction in the temperature of solution or by drastically increasing the CO2 solubility for imparting the antisolvent effect. Most of these CO2 processes require high-pressure pumps, specially designed nozzles and accurate control of process parameters. In order to obviate these requirements, a simple technique of precipitation by pressure reduction over the gas-expanded liquids (PPRGEL), such as CO2-expanded organic solutions has been utilized to impart a large, uniform and rapid reduction of temperature in the solution for instantaneous precipitation of ultra-fine particles. This process utilizes sub-critical CO2 at relatively low pressures of 40–70 bar and near ambient temperature of 303 K for creating a temperature drop of 30–70 K in the solution within seconds, without using any specially designed nozzle or high-pressure pumps. The present paper validates the process principle for precipitation of Zinc acetate (ZnAc) nanoparticles from its organic solution in a mixed solvent of acetone and dimethyl sulfoxide (DMSO). Nanoparticles are produced with the average size of 20–250 nm (from 100 ml of solution in a high-pressure vessel of 1.09 L working volume), and vary in shapes such as long needles, rods and near spherical depending on pressure (40–70 bar at 303 K), solid concentration (0.01–0.05 g/ml) and addition of stabilizer.

Graphical abstractNanoparticles of zinc acetate (ZnAc) have been produced from its solution in a mixed solvent of acetone and dimethyl sulfoxide (DMSO) by employing the recently developed PPRGEL process using sub-critical carbon dioxide (CO2). The process creates rapid, high and uniform supersaturation of solid solute by drastic temperature reduction in the solution as CO2 liberates by rapid pressure reduction. Nanoparticles with the average size of 20–250 nm are produced and vary in shapes such as long needles, rods and near spherical with change in process parameters.Figure optionsDownload full-size imageDownload as PowerPoint slide