Impact of Salt Ions on the Structure and Dynamics of Water

Water is a fundamental substance in characterizing many biological and chemical processes. Despite its simple appearance, water possesses a wide variety of dynamic and structural anomalies that still elude complete understanding. Since water is often found in ion solutions, studying the effects of ions on the anomalies and structure and dynamics of water is of great interest. With computer simulations of NaCl aqueous solution, we found that ions have different effects on water’s structure and dynamics at different temperatures. At room temperature, Na+ and Cl- ions retard water's motion by electrostatic drag. At lower temperatures, ions facilitate water’s movement within a medium range by breaking the immobile locally favored structures. The former effect supports the traditional classification of NaCl as a structure maker, whereas the latter leads to its “unusual” structure-breaker nature at low temperatures. Our results reveal the dual effects of ions on water’s structure and dynamics, which challenges the old concept of structure maker and breaker traditionally defined at room temperature, and advances our understanding of aqueous solutions in a wide range of temperature and salt concentration.