Ion Hydration and Associated Defects in Hydrogen Bond Network of Water: Observation of Reorientationally Slow Water Molecules Beyond First Hydration Shell in Aqueous Solutions of MgCl$_2$

摘要

Effects of presence of ions, at moderate to high concentrations, on dynamicalproperties of water molecules are investigated through classical moleculardynamics simulations using two well known non-polarizable water models.Simulations reveal that the presence of magnesium chloride (MgCl$_2$) inducesperturbations in the hydrogen bond network of water leading to the formation ofbulk-like domains with \textquoteleft defect sites\textquoteright~on boundariesof such domains: water molecules at such defect sites have less number ofhydrogen bonds than those in bulk water. Reorientational autocorrelationfunctions for dipole vectors of such defect water molecules are computed atdifferent concentrations of ions and compared with system of pure water.Earlier experimental and simulation studies indicate significant differences inreorientational dynamics for water molecules in the first hydration shell ofmany dissolved ions. Results of this study suggest that defect water molecules,which are beyond the first hydration shells of ions, also experiencesignificant slowing down of reorientation times as a function of concentrationin the case of MgCl$_2$. However, addition of cesium chloride(CsCl) to waterdoes not perturb the hydrogen bond network of water significantly even athigher concentrations. This difference in behavior between MgCl$_2$ and CsCl isconsistent with the well-known Hofmeister series.