Rotational Relaxation in Supercritical Carbon Dioxide Revisited: A Study of Solute-Induced Local Density Augmentation

Abstract

We examine the rotational diffusion of copper 2,2,3-trimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedionate in liquid solvents and supercritical carbon dioxide using electron paramagnetic resonance spectroscopy. We find that rotational correlation times in the CO2 were considerably larger than those predicted by Stokes−Einstein−Debye theory at regions close to 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 apparent local density approaches that of the bulk. Although consistent with 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 dynamics approach, the enhancement is less than that shown experimentally and compares reasonably to liquid densities. A clear maximum for this enhancement at subcritical densities is shown for the first time in these types of simulations. Reaction rate constants for Heisenberg spin exchange 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 rotation in supercritical CO2.