Do ions affect the structure of water? The case of potassium halides

We study the structural properties of aqueous solutions of potassium chloride and fluoride at ambient conditions and upon supercooling with the use of molecular dynamics. We perform the calculations at increasing concentration in a range from 0.67 mol/kg up to 3.96 mol/kg. We investigate the modifications of the hydration shells and the changes in the water structure induced by the presence of the ions. The oxygen-oxygen structure is strongly dependent on the ionic concentration while the hydrogen bonding is well preserved. The results show that increasing the ion concentration is analogous to increase pressure on pure water. The KF salt has a stronger effect on water due to the fluoride anion. F− appears also more effective as substitutional of the oxygen in the water network with possible relevant consequences on the tendency of water to transform to the high density phase.