Cooperativity in Ion Hydration

Wet Twists and Turns When salts dissolve in water, their constituent positively and negatively charged ions are pulled apart and surrounded by shells of H 2 O molecules (see the Perspective by Skinner ). Ji et al. (p. 1003 ) looked closely at the motion in these shells, using a type of vibrational spectroscopy sensitive to both the orientation and to the neighbors of the targeted molecules. In agreement with recent theoretical predictions, the individual water molecules shifted orientation between an anion and the surrounding liquid in sudden discrete steps, rather than by making smooth incremental rotations. Tielrooij et al. (p. 1006 ) compared the relative impacts of cations and anions on the rigidity of the wider water network, using spectroscopic techniques sensitive to the role of each ion. Certain cation/anion combinations, such as magnesium sulfate, appeared to act together to restrict water motion beyond the boundaries of individual shells.