Influence of thermodynamic behaviour and solute properties on homogeneous nucleation in supercritical solutions

The knowledge about the thermodynamic behaviour of dilute supercritical solutions is one of the basics for modelling processes, such as the formation of small particles by rapid expansion of supercritical solutions (RESS). RESS allows the production of particles less than 1 mum and RESS experiments show that particle size depends on solvent, solute and preexpansion conditions. However, an understanding of the underlying physical phenomena of the relationship between the process conditions and the mechanism of particle formation during RESS is still at an early stage. Because of that, there is a need to model the RESS process to get a better understanding of the influencing parameters. The calculations show a steep increase at the beginning of the freejet reaching maximum theoretical supersaturations of approximate to 10(8) and for an interfacial tension of 0.02 N m(-1) maximum nucleation rates of about 10(+26) (cm(-3) s(-1)). In the present paper, the influence of the solubility of various solutes in supercritical fluids and of the surface tension group (sigma . v(s)(2/3)/k . T) of the diverse solutes on attainable nucleation rates under typical RESS operation conditions is investigated. The calculations show that the nucleation rate is a sensitive function of the solubility and of the unknown surface tension group. Furthermore, it is shown that the classical nucleation theory is not able to describe the trend in particle size resulting from RESS experiments in a sufficient manner. Also, the present calculations show that it is not possible to investigate homogeneous nucleation and coagulation separately and that there is an enormous need for more reliable information about the solute properties. (C) 2000 Elsevier Science B.V. All rights reserved.