The isobaric acid/base equilibrium between acridine and the acridinium cation was measured from ambient temperature to 380°C (above the critical temperature of water, T c ) 374°C) using absorption spectroscopy. At 3500 psia, the isobaric protonation of acridine is shown to be exothermic up to approximately 315°C. Above 315°C, protonation becomes endothermic due to changes in the dielectric constant of water with temperature, which has a profound influence on the solvation of ions. The results are interpreted using a modified Born equation to account for the temperature-dependent changes in acridinium cation and proton solvation. The absorption and fluorescence spectra and the fluorescence lifetime of acridine are sensitive to changes in solvent-solute hydrogen bonding. Hydrogen bonding between acridine and water is observed to decrease from ambient temperature to the critical temperature. A relatively rapid change in hydrogen bonding occurs between 100 and 200°C.Acridine in Subcritical and Supercritical Water

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A kinetic characterization of the excited-state deprotonation of P-naphthol in sub-and supercritical water establishes large deviations from Arrhenius behavior above 110 "C. The fluorescence decay rate constant increases much less with temperature than expected and then decreases at temperatures and pressures beyond the critical point. The loss of solvation of water above 200 "C strongly inhibits its ability to accept a proton and accelerates the reprotonation rate. Under basic conditions, where water clustering and reprotonation are much less important, the observed decay rates deviate much less significantly from the expected Arrhenius behavior. Above the critical point, the relaxation rate constants exhibit strong pressure dependence and are linearly related to the square of the solution density. In contrast with the absence of shifts in the absorption spectrum, pressure-induced shifts in the observed emission maxima near the critical region are assigned to the involvement of contact ion pairs derived from partially deprotonated molecules, and emission from the excited state of the naphtholate-potassium [NapO--K+] ion pair is observed in KOH solution at 200 "C.

The isobaric rates of excited-state deprotonations of 2-naphthol by acetate and borate anions exhibit only modest deviations from Arrhenius-like behavior from ambient temperature to nearly the critical temperature of water (T c = 374 °C). In contrast, the rates of deprotonation by ammonia and water exhibit marked deviations from Arrhenius-like behavior and go through a maximum at high temperatures. These observations establish a fundamental difference in how the rates of charge-generating reactions, such as proton transfers to neutral molecules like ammonia and water, and those in which ionicity is unchanged, such as proton transfers to acetate and borate anions, depend on temperature. The loss of local water structure and changes in dielectric constant with temperature have a much more profound influence on the charge-generating reactions. These results are interpreted using transition state theory and compared with several molecular dynamics−free energy perturbation simulations. At temperatures above 250 °C, contact ion pair formation further inhibits deprotonation. The formation of contact ion pairs is evident in both the time-resolved fluorescence and steady-state fluorescence spectra. Near the critical point, where solvent properties vary widely with pressure, the bimolecular rate constant for 2-naphthol deprotonation by ammonia increases by nearly an order of magnitude over the pressure range from 3000 to 5000 psia. This effect is caused by the large changes in solvent density induced by pressure changes and leads to electrostriction about the polar transition state.

The pH of aqueous sulfuric acid and sulfuric acid−ammonia mixtures was measured using the optical indicator acridine at temperatures from 200 to 400 °C and pressures from 3500 (24.1) to 6000 psia (41.3 MPa). Because of large changes in the pK a of protonated acridine in supercritical water (SCW), the measurable pH range shifts from 2−4 at a density of 0.60 g/cm3 to 4.5−7 at a density of 0.24 g/cm3. At 3500 psia, the first dissociation constant (K a1) of H2SO4 decreases sharply with increasing temperature above 350 °C, primarily due to a reduction in density and thus the solvation of the bisulfate and hydrogen ions. The acidity of H2SO4 relative to HCl increases with increasing temperature at constant pressure up to the critical point of pure water. Based on titrations of sulfuric acid solutions with ammonia, weak acid−weak base behavior is observed at 380 °C and 5000 psia (34.5 MPa). At these conditions the system H2SO4−NH4HSO4 may be used as a buffer to maintain pH in the range 3.5 ± 0.25.