Supercritical Fluids Assisted Nano-Particles Processing

Abhijit Gokhale

Advisor: Dr Boris Khusid

Supercritical fluids (SCF) offer a “solvent-free” approach for the production and coating of nanometer and sub-micrometer particles by the elimination of environmentally harmful organic solvents. Furthermore, SCF-technologies enable the fabrication of powders with properties that are difficult or even impossible to achieve by traditional methods like milling, crystallization, or spray drying. In the Supercritical Anti-solvent (SAS) process, an organic liquid solution of the polymer of interest is injected through a nozzle into a chamber containing a highly compressed gas or supercritical fluid (inexpensive and environmentally benign carbon dioxide is a common choice), which is miscible with the solvent, but is an antisolvent for this polymer.

Recently published studies of the SAS formation of polymer particles are limited to the use of good organic solvents or their mixtures and relatively wide nozzles. In contrast, we use our new concept of mixing “good” and “poor” solvents and micro-nozzles [1] to regulate the precipitation of a polymer from the injected solution. A high-speed CCD camera coupled with a dual laser was used to study the formation of Polyvinylpyrrolidone (PVP) particles in PVP solutions injected into supercritical CO₂. Fused silica capillaries of 10- , 20- , 40- , and 127- um were used as micro-nozzles to inject polymer solutions. Binary mixtures of dichloromethane (DCM) and acetone, which are respectively “good” and “poor” PVP solvents, were used to prepare PVP solutions. The Raleigh and Weber regimes of the jet breakup were found to suppress the particle aggregation and, therefore, favor the formation of polymer nano-powders, having narrow particle size distribution. Increasing the acetone content in the solvent was also shown to suppress the particle aggregation, decrease the average particle size, and eliminate the surface irregularities of the PVP particles. However, increasing the operational pressure well above the critical point was found to increase the average size of the PVP particles and broaden the particle size distribution.

1. A. Gokhale, B. Khusid, R. Dave, R. Pfeffer, Formation of polymer nano-particles in supercritical fluid jets, NSTI Nanotechnology, v. 1, 2005, 215-217.