A classical molecular dynamics study of the anomalous ionic product in near-critical and supercritical water

Abstract

This paper reports a molecular dynamics simulation study on the ionic dissociation of water. Experimentally the ionic product of water, KW , is known to exhibit an anomalous variation with temperature. It rises to a maximum at near-critical temperatures before falling off sharply as the temperature becomes super-critical. The paper describes a molecular dynamics simulation study, using the SPC/E water model, to gain insights into this surprising behaviour. Results for KW are in good agreement with experiment up to the critical point and the anomalous trend appears to be the result of a balance between the effects of a positive gas phase enthalpy of reaction, a decreasing electrostatic stabilisation of the ions with increasing temperature and more subtle entropic effects. The possibility of ion-pairing effects are considered and a simple estimate of the equilibrium dimerisation constants shows the dimer to be preferred at low temperatures, and the separated ions to be increasingly favoured at higher temperatures. The experimental value of pKW was fitted to simulation results at 700 K to give an estimated water dipole of 2.06 Debye.