ROTATIONAL RELAXATION IN SUPERCRITICAL CARBON-DIOXIDE REVISITED - A STUDY OF SOLUTE-INDUCED LOCAL-DENSITY AUGMENTATION

We examine the rotational diffusion of copper methyl-6,6,7,7,8,8,8-hep tafluoro-3,5-octanedionate in liquid solvents and supercritical carbon dioxide using electron paramagnetic resonance spectroscopy. We find t hat rotational correlation times in the CO2 were considerably larger t han those predicted by Stokes-Einstein-Debye theory at regions close t o the critical density, a finding similar to that of Heitz and Bright. Local density augmentation was quantified using a model developed by Anderton and Kauffman. At low bulk densities, we find extraordinarily high local density enhancements, with local densities over four times higher than those of the bulk, while at higher bulk densities, the app arent local density approaches that of the bulk. Although consistent w ith the results reported by Heitz and Bright, apparent local densities far surpassed those of the liquid solvents, a physically unreasonable result. When local densities are calculated using a molecular dynamic s approach, the enhancement is less than that shown experimentally and compares reasonably to liquid densities. A clear maximum for this enh ancement at subcritical densities is shown for the first time in these types of simulations. Reaction rate constants for Heisenberg spin exc hange from experiments are found to be consistently higher than those predicted by theory, indicating a likely contribution of solute-solute local density enhancements to the observed slowing of molecular rotat ion in supercritical CO2.