STATE-DEPENDENT SOLVATION OF PYRENE IN SUPERCRITICAL CO2

We experimentally address the issue of solute-fluid and solute-solute interactions in dilute supercritical solutions. Using the fluorescent probe pyrene, we carry out a detailed spectroscopic study of pyrene-py rene and pyrene-fluid interactions as a function of CO2 density. UV-vi s absorbance and steady-state fluorescence experiments confirm that th e excimer-like emission seen for pyrene is not the result of soluble p yrene aggregates nor does it result from detectable, dissolved ground- state pyrene species. The extent of local density enhancement surround ing pyrene molecules is also independent (within experimental error) o f pyrene concentration for the ground- and excited-state species, indi cating that the pyrene molecules are solvated individually, not in pai rs or higher aggregates. The magnitude of local density enhancement su rrounding an excited-state pyrene molecule is, however, 1.5 times that of the ground-state pyrene species. This particular result is in exce llent agreement with predictions based on the electrostatic interactio n energy of ground- and excited-state pyrene with CO2. Together these results provide strong evidence against dissolved ground-state solute- solute preassociation between pyrene molecules in CO2 and provides the first experimental evidence on state-dependent local density enhancem ents in dilute supercritical fluid solutions.