Structure and dynamics of ion hydration shells
The dynamics of water molecules in aqueous solvation shells of ions can be studied with femtosecond mid-infrared vibrational spectroscopy and GHz-THz dielectric relaxation spectroscopy. This combination of techniques allows the probing of the reorientation of water along different axes of the molecule. Recently, we found that ions affect the dynamics of water in a highly anisotropic manner: cations strongly slow down the reorientation of the water dipoles, while anions slow down the reorientation of the O-H groups. For most ions the slowing down effect is restricted to the first structural hydration shell. However, for specific ion combinations, like the strongly hydrating ions Mg2+ and SO42-, the influence of the ions is surprisingly long-range and extends well beyond the first structural hydration shells. For these combinations many water molecules are slowed down (17 per Mg2+ + SO42- Mg2+ + SO42-). Interestingly, the effect is also much larger than the sum of the separate ion contributions (determined by combining the separate ions with other (weakly hydrating) ions). This observation has implications for understanding Hoffmeister effects of salts on proteins. In the future we will continue the study of the effects of ions on the structure and dynamics of water.
Schematic picture of water hydrating a cation (above) and an anion (under). For the cation, the water dipoles are radially pointing away from the positive charge. The reorientation of the dipoles is strongly slowed down but the O-H groups can rotate around the dipolar axis with a rate similar to that of the reorientation of water molecules in the bulk liquid. For the anion one of the two O-H groups of the water molecule is strongly interacting with the negative charge. The reorientation of this O-H group is slowed down with respect to bulk liquid water. The other O-H group shows a reorientation rate that is similar to that of water molecules in the bulk.
