CHEMICAL PHYSICAL INTERPRETATION OF COSOLVENT EFFECTS IN SUPERCRITICAL FLUIDS

Cosolvent effects in supercritical fluids can be considerable for systems where the cosolvent interacts strongly with the solute. It is shown in a companion paper that strong interactions occur between naproxen ((S)-6-methoxy-alpha-methyl-2-naphthaleneacetic arid) and various polar cosolvents in supercritical CO2. A chemical-physical model proposed by Ekart and Eckert that incorporates chemical equilibria into the Soave-Redlich-Kwong equation of state was used here to study these systems. The model was found to describe these systems well, and the associated equilibrium constants obtained could be correlated with the cosolvent alpha, beta, and pi* solvatochromic parameters by a linear free energy relationship. This correlation suggests that both physical and chemical forces are important in the solvation process of naproxen in polar cosolvent-supercritical CO2 mixtures. The model coupled with the correlation represents a step toward predicting solubilities in cosolvent-modified supercritical fluids using nonthermodynamic data. This method of modeling cosolvent effects allows a more intuitive interpretation of the data than either a purely physical equation of state or ideal chemical theory can provide.