The use of a supercritical Solvent (S)–Antisolvent (AS) process (SAS) for fine particle production is finding widespread industrial applications. The
perfection of this technology requires insight into many basic laws of interface and colloid science. In SAS the solute is dissolved in an organic solvent and the solution is sprayed into a near critical AS stream. SAS is a complex process involving the interaction of jet hydrodynamics, droplet formation, mass transfer, phase equilibrium, intra-droplet nucleation, and microcrystal growth. A complete description would have to take into account all of these processes; however, such a model is not currently available. In the two-phase flow of an S/AS emulsion, S diffuses from droplets into AS, while AS dissolves inside the S droplets. S replacement by AS (Supercritical CO
2) causes solute supersaturation in the droplets. When it occurs near the critical point of the S/AS emulsion (80 bar, 32 °C), intra-droplet nucleation and precipitation of the solute occurs.
The possibility of solute particle production and the particle size is controlled by the droplet size and by the interrelationship between three time scales. These are the droplet mass transfer time τN, the nucleation time τN, i.e., the time necessary for one particle nucleus to form in one droplet, and the droplet residence in the supersaturated stream τres.
An approximate analytical theory for intra-droplet nucleation is developed and the conditions necessary for nanoparticle production are established. The smaller the droplet dimension and the lower the solute concentration, the smaller the particle dimension that is obtained. The recent success in membrane emulsifying may be used for the production of micron-sized droplets. After the AS stream is saturated with S due to partial dissolution of the droplets, a quasi-equilibrium between the droplets and AS stream occurs and a steady and uniform zone with intra-droplet supersaturation is formed downstream. But τres > τN is necessary for one nucleus formation per droplet, i.e., τres has to be much longer than that reported in the literature (10− 3 s), because τN increases with decreasing droplet dimension. Accordingly, a long residence time version of the SAS process (τres ![]()
1 s) is necessary. However, a long τres is problematic because of micro-droplet turbulent coagulation. Since an increase in τres is difficult, a decrease in τN by means of an increase in S becomes significant. This is achieved by using a phenomenon which we call supersaturation of the second kind S2. In the literature attention is paid only to a decrease in the equilibrium solute concentration, when solvent and antisolvent are mixed. However, S2 occurs due to an actual increase in concentration of solute within the droplets as they shrink due to S dissolution. The smaller the ratio of solvent to antisolvent flow rate, the larger the droplet shrinkage and the higher the S2 achieved. Due to large S2, nanoparticle production becomes possible even for solutes with high surface tension σ and large molecular volume Vo, while earlier it was impossible because of the exponential increase of τN with increasing Vo and σ. Combining a long τres and variable and precisely controllable supersaturation, which is uniform in space and enhanced due to S2, creates an opportunity for standardization of characterizing different solutes through their τN, which is the key solute property affecting nanoparticle production by SAS.
