Role of Charge Transfer in Water Diffusivity in Aqueous Ionic Solutions

We performed molecular dynamics simulations on four types of systems containing ion and solvating water. Two systems contained a cation (Na+ or K+), and two other systems an anion (Cl- or I-). Classical molecular dynamics simulations were performed using three different force fields: a fixed charge force field, a polarizable force field that includes explicit polarization, and also a recently developed force field that includes polarization and charge transfer. These simulations were then compared to first-principles molecular dynamics simulations. While the first-principles simulations showed that the anions accelerated water translational diffusion, the cations slowed it down. In simulations with the classical force fields, only the force field that incorporates explicit charge transfer reproduced this ion-specific behavior. Additional simulations performed to understand the effect of charge transfer demonstrated that two competitive factors determine the behavior of water translational diffusion: the ions diminished charge accelerates water, while the net charge acquired by water either accelerates or slows down its dynamics. Our results show that charge transfer plays a crucial role in governing the water dynamics in aqueous ionic solutions.