Benzophenone as a probe of local cosolvent effects in supercritical ethane

The n --> pi* shift of benzophenone has been used to quantify solute-cosolvent interactions in supercritical ethane. Dilute solutions of benzophenone in cosolvent/supercritical ethane mixtures were studied at 35 degrees C from 50 to 100 bar over a range of cosolvent concentrations. The following cosolvents were chosen for investigation on the basis of their varying abilities to interact with benzophenone: 2,2,2-trifluoroethanol, ethanol, chloroform, propionitrile, 1,2-dibromoethane, and 1,1,1-trichloroethane. In the supercritical systems investigated here, hydrogen bonding of protic cosolvents to the carbonyl oxygen of benzophenone is the primary mechanism of the n --> pi* shift. The results of this investigation are consistent with a chemical-physical interpretation of cosolvent effects in supercritical fluids in the presence of strong specific solute-cosolvent interactions. The experimental results for the ethane/TFE/benzophenone system were analyzed by using integral equations in order to study the assumptions of the chemical-physical model. This combination of spectroscopic data with radial distribution function models provides a powerful tool for understanding cosolvent effects.