Simulations of Acid Dissociation Constants of Polyprotic Acids in Near-Critical and Supercritical Water

摘要

This paper reports a molecular dynamics study on the dissociation of sulfuric acid and phosphoric acid in near-critical and supercritical water. pK_a is known to vary as the temperature and pressure vary, and this variation has important implications for corrosion in supercritical water reaction vessels. This work uses the SPC/E water model and solutes based upon DFT calculations to examine both structural and thermodynamic properties of the dissociation processes. An increase in solute—solvent ordering is observed for larger charges, and this also corresponds to a lower rate of diffusion. All dissociation reactions become less favored with increasing temperature except pK_(a1) for sulfuric acid which becomes significantly more favored until 748 K.