Analysis of retrograde behavior and the cross-over effect in supercritical fluids

In this paper, results based upon thermodynamic stability theory are developed which lead to a set of both necessary and sufficient conditions for the existence of retrograde behavior in a multicomponent solute system dissolved in a pure supercritical fluid. While experimental evidence of retrograde behavior in single solute systems has been known for some time, recently data have been obtained showing the retrograde effect in binary solute systems dissolved in a pure supercritical fluid.In such systems, cross-over regions may be defined. These are pressure—temperature regions where the solubility of one solute increases while that of the other decreases with a change in temperature at constant pressure. The existence of cross-over regions in multicomponent mixtures can have implications for a new separation technique using pure supercritical fluids. In conjunction with an equation of state, the results derived here allow cross-over regions to be predicted, thus enabling one to identify candidate systems and thermodynamic conditions for the cross-over process. For this work a variation of a perturbed hard sphere model equation of state was used for the calculations.