Spectroscopic study of structure and interactions in cosolvent-modified supercritical fluids

Cosolvents can be used to tailor supercritical fluid solvents for increased selectivity by increasing the solvent polarity or polarizability and participating in specific interactions with solutes. Specific interactions in these fluids are often inferred from thermodynamic measurements although independent confirmation of the structure of cosolvent/solute interactions is lacking. The solutes 2-naphthol, 5-cyano-2-naphthol, and 7-azaindole exhibit well-characterized spectral responses to an organized, hydrogen-bonded solvent environment in liquids. We describe fluorescence and UV absorption experiments to probe the structure and interaction between polar cosolvents and these solutes in a variety of supercritical fluids. In addition, new solubility data are reported for 7-azaindole in supercritical CO₂ with and without methanol and in supercritical ethane. The results suggest that the interaction between cosolvent and solute can be strong yet exhibit no evidence for actual complexes in solution.