Binary structure and dynamics of the hydrogen bonds in the hydration shells of ions

Ion-specific effects of cations (Li+, Na+, K+, Mg2+, Ca2+) and anions (F-, Cl-) on the hydrogen bond structure and dynamics of the coordination waters in the hydration shells have been studied using molecular dynamics simulations. Our simulations indicate that the hydrogen bonds between the first and second hydration shell waters show binary structural and dynamic properties. The hydrogen bond with a first shell water as the donor (HD) is strengthened, while those with a first shell water as the acceptor (HA) are weakened. For a hydrated anion, this binary effect reverses, but is less significant. This ion-specific binary effect correlates with the size and the valence of the ion, and is more significant for the strong kosmotropic ions of high charge density.