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+.

Y.L.A. Rezus (Yves)
Phys. Chem. Chem. Phys.
Ultrafast Spectroscopy

Pastorczak, M., van der Post, S. T., & Bakker, H. (2013). Cooperative hydration of carboxylate groups with alkali cations. Phys. Chem. Chem. Phys., 15, 17767–17770. doi:10.1039/C3CP52419A