BENZOPHENONE AS A PROBE OF LOCAL COSOLVENT EFFECTS IN SUPERCRITICAL ETHANE

The n --> pi shift of benzophenone has been used to quantify solute-c osolvent interactions in supercritical ethane. Dilute solutions of ben zophenone in cosolvent/supercritical ethane mixtures were studied at 3 5 degrees C from 50 to 100 bar over a range of cosolvent concentration s. The following cosolvents were chosen for investigation on the basis of their varying abilities to interact with benzophenone: 2,2,2-trifl uoroethanol, ethanol, chloroform, propionitrile, 1,2-dibromoethane, an d 1,1,1-trichloroethane. In the supercritical systems investigated her e, hydrogen bonding of protic cosolvents to the carbonyl oxygen of ben zophenone is the primary mechanism of the n --> pi shift. The results of this investigation are consistent with a chemical-physical interpr etation of cosolvent effects in supercritical fluids in the presence o f strong specific solute-cosolvent interactions. The experimental resu lts for the ethane/TFE/benzophenone system were analyzed by using inte gral equations in order to study the assumptions of the chemical-physi cal model. This combination of spectroscopic data with radial distribu tion function models provides a powerful tool for understanding cosolv ent effects.