Watching the low-frequency motions in aqueous salt solutions: The terahertz vibrational signatures of hydrated ions

The details of ion hydration still raise fundamental questions relevant to a large variety of problems in chemistry and biology. The concept of water "structure breaking" and "structure making" by ions in aqueous solutions has been invoked to explain the Hofmeister series introduced over 100 years ago, which still provides the basis for the interpretation of experimental observations, in particular the stabilization/destabilization of biomolecules. Recent studies, using state-of-the-art experiments and molecular dynamics simulations, either challenge or support some key points of the structure maker/breaker concept, specifically regarding long-ranged ordering/disordering effects. Here, we report a systematic terahertz absorption spectroscopy and molecular dynamics simulation study of a series of aqueous solutions of divalent salts, which adds a new piece to the puzzle. The picture that emerges from the concentration dependence and assignment of the observed absorption features is one of a limited range of ion effects that is confined to the first solvation shell.