Rotational relaxation in a nondipolar supercritical fluid: Toluene in CO2

Studies of rotational relaxation dynamics provide particular insight into l ocal solution structures and consequently into the interactions between spe cies in a solution. We report here the results of molecular dynamics simula tions describing a neat CO2 supercritical fluid and an infinitely dilute so lution of toluene in supercritical CO2. Over a period of 0.1-0.2 ps, the ro tation of the near-critical solvent molecules is relatively unhindered, bec oming purely diffusive only on a time scale that is long compared with the decay of the orientational correlations. As expected, the rotational relaxa tion rate of a toluene molecule is found to increase with increasing solven t density, although the simulation results imply some anomalous behavior ne ar the critical point that may be associated with the appearance of long-ra nge spatial correlations. We also show that a system consisting of a nonpol ar toluene analogue experiences an isotropic rotational friction environmen t, unlike the anisotropic environment in which a real toluene molecule is f ound when dissolved in supercritical CO2.