ROTATIONAL RELAXATION IN THE COMPRESSIBLE REGION OF CO2 - EVIDENCE FOR SOLUTE-INDUCED CLUSTERING IN SUPERCRITICAL-FLUID SOLUTIONS

Rotational correlation times of two solutes, trans, trans-1,4-diphenylbutadiene (DPB) and trans-4-(hydroxymethyl)stilbene (HMS), have been measured in supercritical CO2 as a function of density at a reduced temperature of 1.01 (35 degrees C) across the compressible region. The DPB rotational correlation time increases by 50% upon increasing density from 0.3 to 0.8 g/cm(3), whereas the HMS rotational correlation time increases by more than 300% over the same density regime. The results are compared with a model that is developed to describe the influence of local solvent density on the rotational correlation time of the solute. The model predicts a strong dependence of the rotational correlation time on the local solvent density as the liquid density of CO2 is approached. On the basis of a comparison of the data with the model, the experimental results are interpreted as evidence for solute-induced clustering of CO2 around HMS as a result of attractive solute-solvent interaction. The relative magnitude of the observed effect in HMS compared with DPB is taken as evidence that the operative interaction occurs between CO2 and the hydroxyl group of HMS.