Control of Powders Morphology in the Supercritical Antisolvent Technique Using Solvent Mixtures

The supercritical antisolvent process (SAS) has been frequently used to obtain microparticles and nanoparticles. The fluid dynamics of the process related to the study of the liquid jet in contact with supercritical carbon dioxide (scCO2) is characterized by a one-phase mixing at supercritical conditions and a two-phase mixing at subcritical conditions. The transition between the two kinds of mixing can be measured in terms of amplitude of the corresponding pressure range; some organic solvents, like dimethylsulfoxide (DMSO) are characterized by a wide pressure range, other solvents, like acetone (AC), by a narrow pressure range. Generally, microparticles are precipitated by atomization, droplets formation and drying in the transition range, whereas nanoparticles are precipitated in correspondence of completely developed supercritical conditions. Mixing a wide-transition solvent, like DMSO, to a narrow-transition solvent, like acetone, the pressure range of the transition from one-phase mixing to two-phase mixing and, accordingly, the morphology of the precipitates will change. In this work, two model compounds were SAS processed from DMSO/AC mixtures: cellulose acetate, which is slightly soluble in DMSO and freely soluble in acetone with the aim of obtaining microparticles and polyvinylpyrrolidone (PVP) that is slightly soluble in acetone and freely soluble in DMSO in order to obtain nanoparticles. In the case of cellulose acetate, well-defined microparticles with a mean diameter of 0.42 μm were obtained, whereas, for PVP, nanoparticles with a mean diameter of 114 nm were precipitated, demonstrating that this SAS strategy is successful.