Hydration numbers from ab initio water reorientation dynamics

We present a novel approach to determine the hydration number of aqueous electrolytes by means of ab initio molecular dynamics. An hydration status analysis is devised to quantify the cooperative effect of ions on the reorientational dynamics of different water subpopulations in electrolyte solutions. The methodology is applied to predict the hydration numbers, h, as the number moles of water molecules per mole of dissolved salt that no longer participate in bulk-like reorientational dynamics. The obtained hydration number of MgCl2 (h = 15) is consistent with THz-DR experiment, measuring reorientational dynamics of water in solutions. By providing a link with the micro-scale dynamical behaviour of ions and water molecules, this approach represents a generally applicable, welldefined methodology to quantify hydration numbers of ions and molecules in aqueous solutions.