Ion segregation in aqueous solutions.

Microscopic structures and dynamics of aqueous salt solutions were investigated with the ultrafast vibrational energy exchange method and anisotropy measurements. In KSCN aqueous solutions of various concentrations, the rotational time constants of SCN(-) anions are proportional to the viscosities of the solutions. However, the reorientation dynamics of the water molecules are only slightly affected by the solution viscosity. With the addition of strongly hydrated F(-) anions, the rotations of both SCN(-) anions and water molecules slow down. With the addition of weakly hydrated I(-) anions, only the rotation of SCN(-) anions slows down with that of water molecules unaffected. Vibrational energy exchange measurements show that the separation among SCN(-) anions decreases with the addition of F(-) and increases with the addition of I(-). The series of experiments clearly demonstrate that both structures and dynamics of ion and water are segregated in the strong electrolyte aqueous solutions.