Ab Initio Study on Molecular and Thermodynamic Properties of Water: A Theoretical Prediction of pKw over a Wide Range of Temperature and Density

Abstract

The ionic product of water (pKw) has been calculated in a wide range of temperature (0−600 °C) and density (0.6−1.4 g/cm3) by means of ab initio electronic structure theory combined with the extended reference interaction site model in statistical mechanics for molecular liquids (RISM-SCF/MCSCF). We consider the autoionization process (H2O + H2O ⇌ H3O+ + OH-) by regarding H2O, H3O+, and OH- as “solute” molecules in an aqueous solution and evaluate molecular geometries, electronic structure, solvation structure, and free energy components of these species as functions of thermodynamical conditions. The results for pKw obtained from the theory have shown a monotonical decrease with increasing density at all the temperatures investigated, in good accord with the experimental observation. The behavior is determined essentially by the difference in solvation free energies, Δμ(H3O+) + Δμ(OH-) − 2Δμ(H2O), associated with the reaction. The Δμ(OH-) shows the density dependence, which is entirely different from tha...