Hydrogen bond and residence dynamics of ion-water and water-water pairs in supercritical aqueous ionic solutions: dependence on ion size and density

Abstract

We have carried out a series of molecular dynamics simulations to investigate the hydrogen bond and residence dynamics of X^--water (X = F, Cl, and I) and pairs in aqueous solutions at a temperature of 673 K. The calculations are done at six different water densities ranging from 1.0 to 0.15 g cm^-3. The hydrogen bonds are defined by using a set of configurational criteria with respect to the anion(oxygen)-oxygen and anion(oxygen)-hydrogen distances and the anion(oxygen)-oxygen-hydrogen angle for an anion(water)-water pair. The F^--water hydrogen bonds are found to have a longer lifetime than all other hydrogen bonds considered in the present study. The lifetime of Cl^--water hydrogen bonds is shorter than that of F^--water hydrogen bonds but longer than the lifetime of water-water hydrogen bonds. The lifetimes of I^--water and water-water hydrogen bonds are found to be very similar. Generally, the lifetimes of both anion-water and water-water hydrogen bonds are found to be significantly shorter than those found under ambient conditions. In addition to hydrogen bond lifetimes, we have also calculated the residence times and the orientational relaxation times of water molecules in ion (water) hydration shells and have discussed the correlations of these dynamical quantities with the observed dynamics of anion (water)-water hydrogen bonds as functions of the ion size and density of the supercritical solutions.