Reaction enthalpies along the two channels of geminate electron recombination in liquid-to-supercritical water

• The geminate recombination processes of electrons with hydroxonium ions and hydroxyl radicals, produced by radiation chemical or photochemical methods, are theoretically investigated.
• The enthalpies of the two processes are evaluated for liquid and supercritical water by devising appropriate thermodynamic cycles.
• The hydroxonium process is seen to be highly endothermic, the hydroxyl process highly exothermic at room temperature.
• In supercritical water the difference in the enthalpies is smaller.

Ionizing radiation or UV light produces electrons and H2O+ ions in water. These species transform into hydrated electron, e−aq, hydrated H3O+ ion, and ·OH radical in each other's neighborhood much faster than any forthcoming chemical transformation. Part of the electrons escapes their geminate partners. There exists two possible paths for the remaining fraction to react: H3O++e−aq=H3O· [channel (A)] and ·OH+e−aq=OH− [channel (B)]. We devised two thermodynamic cycles for the computation of the reaction enthalpies of both channels. Channel (A) was found to be endothermic with an enthalpy of 3.61 eV at room temperature. The enthalpy is seen to be almost constant up to 500 K, to increase at 600 K and to drop abruptly around 650 K, i.e. in the region where the dielectric constant is below 20. Channel (B) was found to be exothermic with an enthalpy of −2.33 eV at room temperature. It is becoming gradually less exothermic with increasing temperature the variation becoming fast around 650 K. The tendency of these thermochemical results parallel with recent kinetic calculations by Torres-Alacan et al. (J. Torres-Alacan, S. Kratz, P. Vöhringer, 2011. Phys. Chem. Chem. Phys. 13, 20806–20819)