Rotational relaxation in supercritical CO2

Though there are numerous studies of solute molecule rotational dynamics in ordinary liquids and supercritical fluids, few such studies have focused on the molecules of a neat supercritical fluid. In the present study, we investigate the structure and rotational dynamics of supercritical carbon dioxide at a temperature 1% above its critical temperature and over a 20-fold range of densities about the critical density. Our simulations reveal the presence of density inhomogeneities and thus regions of modest local density enhancement in this fluid over a significant range of bulk densities. Although these inhomogeneities account for the weak bulk density dependence of the CO2 orientational correlation times in the near-critical regime, they are not reflected in a concomitant variation in the observed total rotational friction. Caution is warranted, however, in interpreting the usual diffusive dynamics parameters in this system because only at densities in excess of the critical density may the dynamics be characterized as diffusive; only at lower densities are orientational correlations found to decay on a time scale equal to or shorter than that characterizing the angular velocity correlations.