ABSORPTION AND FLUORESCENCE STUDIES OF ACRIDINE IN SUBCRITICAL AND SUPERCRITICAL WATER

The isobaric acid/base equilibrium between acridine and the acridinium cation was measured from ambient temperature to 380 degrees C (above the critical temperature of water, T-c = 374 degrees C) using absorpti on spectroscopy. At 3500 psia, the isobaric protonation of acridine is shown to be exothermic up to approximately 315 degrees C. Above 315 d egrees C, protonation becomes endothermic due to changes in the dielec tric constant of water with temperature, which has a profound influenc e on the solvation of ions. The results are interpreted using a modifi ed Born equation to account for the temperature-dependent changes in a cridinium cation and proton solvation. The absorption and fluorescence spectra and the fluorescence Lifetime of acridine are sensitive to ch anges in solvent-solute hydrogen bonding. Hydrogen bonding between acr idine and water is observed to decrease from ambient temperature to th e critical temperature. A relatively rapid change in hydrogen bonding occurs between 100 and 200 degrees C.