Microscopic understanding of ion solvation in water

Abstract: Solvation of ions is ubiquitous on our planet. Solvated ions have a profound effect on the behavior of ionic solutions, which is crucial in nature and technology. Experimentally, ions have been classified into "structure makers" or "structure breakers", depending on whether they slow down or accelerate the solution dynamics. Theoretically, the dynamics of ions has been explained by a dielectric friction model combining hydrodynamics and charge-dipole interaction in the continuum description. However, both approaches lack a microscopic structural basis, leaving the microscopic understanding of salt effects unclear. Here we elucidate unique microscopic features of solvation of spherical ions by computer simulations. We find that increasing the ion electric field causes a sharp transitional decrease in the hydration-shell thickness, signaling the ion mobility change from the Stokes to dielectric friction regime. The dielectric friction regime can be further divided into two due to the competition between the water-water hydrogen bonding and ion-water electrostatic interactions: Whether the former or latter prevails determines whether the water dynamics are accelerated or decelerated. In the ion-water interaction predominant regime, a specific combination of ion size and charge stabilizes the hydration shell via orientational-symmetry breaking, reminiscent of the Thomson problem for the electron configuration of atoms. Notably, the hydration-shell stability is much higher for a composite coordination number than a prime one, a prime-number effect on solvent dynamics. These findings are fundamental to the structure breaker/maker concept and provide new insights into the solvent structure and dynamics beyond the continuum model, paving the way towards a microscopic theory of ionic solutions.