Probing Solute-Entrainer Interactions in Matrix-Modified Supercritical CO₂

Binary supercritical fluids composed of CO₂ and small amounts of acetonitrile or methanol (entrainers) are studied as a function of fluid density, using pyrene as the solute probe. These experiments show how preferential entrainer clustering effects pyrene excimer formation. To this end, steady-state and time-resolved fluorescence spectroscopy are used in concert to probe the ground and excited states of pyrene. Results show that entrainers enhance solute-solvent clustering and slow the, traditionally diffusion-controlled, excimer formation reaction. Clustering also helps shield the excimer from non-radiative deactivation processes as evidenced by a longer decay time in the near-critical region. All data over our concentration range are consistent with a homogeneous pyrene ground state. There is no evidence for solute-solute (pyrene-pyrene) interactions prior to excitation. However, once in the excited state, excimer formation is facilitated due to the decreased fluid viscosity compared to liquids.