ON THE LOCAL ENVIRONMENT SURROUNDING PYRENE IN NEAR-CRITICAL AND SUPERCRITICAL WATER

We use steady-state and time-resolved fluorescence spectroscopy to pro be local solvent-solute interactions between pyrene (the solute) and s upercritical water (SCW). Toward this end, we have developed a new fib er-optic-based titanium high-pressure optical cell which can withstand the temperatures and pressure needed to generate supercritical water. Static fluorescence measurements indicate that there is an increase i n the local water density surrounding the pyrene molecules (clustering ) up to five times the bulk fluid density, This extent of clustering i s most prevalent at about one-half the critical density, Consistent wi th previous work on more mild supercritical fluids (e.g., CO2, CF3H, C 2H6), the extent of this solute-fluid clustering decreases as the syst em temperature and pressure are increased. Time-resolved fluorescence measurements show that the excited-state decay kinetics are exponentia lly activated and not themselves affected by this solute-fluid cluster ing process.