Study of solvent-solute and solute-solute interaction effects on naphthalene in carbon dioxide by NMR measurement of rotational and angular momentum correlation times in CO2 at sub- and supercritical temperatures

The effect of temperature variation from subcritical to supercritical conditions on the rotational and angular momentum correlation times of naphthalene dissolved in carbon dioxide is reported. The rotational correlation time, tau(c), is found to be only slightly dependent upon solution viscosity, unaffected by the CO2 phase change at about 32 degrees C and is essentially equal to its value in acetone at the same temperature. tau(c) is approximately twice its gas phase free rotor time. These results are interpreted to indicate that ground state solute-solute interactions are unimportant. The angular momentum correlation time, tau(j), undergoes a dramatic increase at the temperature of the phase change indicating an approximately 3.2. fold solvent density augmentation around the naphthalene compared to the bulk gas phase CO2 density under the experimental conditions beyond the critical temperature. These results indicate that a substantial solvent density augmentation occurs that does not cause measurable changes in the rotational properties of the solute. The solvent augmentation is found to disappear within approximately 2 degrees C on either side of the critical temperature.