Cooperative slowdown of water rotation near densely charged ions is intense but short-range

Abstract

We use classical atomistic molecular dynamics simulations and polarizable models to investigate the dynamics of rotation of room temperature water in the vicinity of magnesium sulfate ions. At infinite Mg2+-SO4 2- separation, both ions strongly slow down the rotation of water in their first hydration shell but only weakly affect molecules in their second hydration shell. At 12 Å Mg2+-SO4 2- separation, a small population of water molecules halfway between the ions rotates more slowly than the equivalent population near the isolated ions, indicating that pairs of anions and cations slow down water rotation in a cooperative manner. Surprisingly, this subpopulation consists exclusively of water molecules in the second or third hydration layers of either ion. In contrast, for a 5 Å Mg2+-SO4 2- separation intense cooperative slowdown of water rotation is observed in the first hydration layer of each ion, and much weaker effects are present in the outer hydration layers. For both small and large Mg2+-SO4 2- distances, cooperative slowdown is larger in multi-ion systems than in systems with a single pair of ions. Our results broadly support the existence of long-range cooperative slowdown of water rotation by ions, initially proposed based on experimental measurements (Tielrooij et al., 2010, Science, 328, 1006), but reveal a much more complex picture of the effect. The dependence of cooperative slowdown on the anion-cation distance, ion concentration and the position of water molecules relative to the ions obtained from simulations can be used to refine the interpretation of experimental measurements to obtain further insight into these systems.