  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.
