Figure 1: Semirigid hydration and cooperativity ((a) and (b)) a water molecule in the solvation shell of a cation (a) and an anion (b). Dielectric relaxation measurements probe the reorientation of the permanent dipole vector p. Femtosecond infrared spectroscopy is sensitive to the reorientation of the OD-stretch transition dipole moment . The dotted arrows indicate reorientation in a cone, in the case of semirigid hydration. (c) Proposed geometry, in which the water dynamics are locked in two directions because of the cooperative interaction with the cation and the anion. Figure 1 is reproduced here from (Tielrooij et al. 2010) [153] with permission of the American Association for the Advancement of Science.