Hydration Structure of Na+, K+, F+, and Cl- in Ambient and Supercritical Water: A Quantum Mechanics/Molecular Mechanics Study

The ion solvation in aqueous solution under different temperatures and pressures governs many important biological, geological, and chemical processes. In this work, we present a theoretical study of hydration structures of several alkali metal cations and halide anions, Na+, K+, F-, and Cl-, in ambient and supercritical water through quantum mechanics/molecular mechanics (QM/MM) simulations. It is shown that our calculated first peak or minimum positions for the radial distribution functions of ion-water pairs and hydration numbers Nhydration as well as hydrogen bond numbers NHB are in good agreements with available experimental determinations or other high-level QM/MM simulated results. We reveal that Nhydration under supercritical condition does not reduce much comparing to that under ambient condition due to the inherent local clustering effect although the bulk density decreases evidently. After the investigation of the angular distribution of oxygen-ion-oxygen within the first hydration sphere, it is shown that Na+ or F- in ambient water tends to be six-fold coordinated in a octahedral-like structure. Through the analysis of ion-oxygen-hydrogen angle distributions within the first hydration spheres of F- and Cl-, we also find that strong hydrogen bonds exist in ambient aqueous F- and become weakened under supercritical condition, whereas weak hydrogen bonds exist in ambient aqueous Cl- and become nearly disappeared under supercritical condition. (C) 2013 Wiley Periodicals, Inc.