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 SUBCRITICAL 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 c orrelation time, tau(c), is found to be only slightly dependent upon s olution viscosity, unaffected by the CO2 phase change at about 32 degr ees C and is essentially equal to its value in acetone at the same tem perature. tau(c) is approximately twice its gas phase free rotor time. These results are interpreted to indicate that ground state solute-so lute interactions are unimportant. The angular momentum correlation ti me, tau(j), undergoes a dramatic increase at the temperature of the ph ase change indicating an approximately 3.2. fold solvent density augme ntation around the naphthalene compared to the bulk gas phase CO2 dens ity under the experimental conditions beyond the critical temperature. These results indicate that a substantial solvent density augmentatio n occurs that does not cause measurable changes in the rotational prop erties of the solute. The solvent augmentation is found to disappear w ithin approximately 2 degrees C on either side of the critical tempera ture.