Cooperative hydration of carboxylate groups with alkali cations

We find that the formate ion leads to the slowing down of the reorientation dynamics of less than 1 water molecule when combined with a weakly hydrating cation like Cs⁺, K⁺, or NH₄⁺. However, up to 3 water molecules are slowed down in their reorientation if the formate ion is combined with a strongly hydrating cation like Na⁺ or Li⁺. These additional slow water molecules are not associated with the hydration shells of the separate cations, but rather are contained in solvent-separated ion pairs formed by the formate ion and Na+ or Li+.