Range of ion specific effects in the hydration of ions

Within the quasichemical approach, the hydration free energy of an ion is decomposed into a chemical term accounting for ion specific ion-water interactions within the coordination sphere and nonspecific contributions accounting for packing (excluded volume) and long range interactions. The change in the chemical term with a change in the radius of the coordination sphere is the compressive force exerted by the bulk solvent medium on the surface of the coordination sphere. For the Na+, K+, F-, and Cl- ions considered here this compressive force becomes equal for similarly charged ions for coordination radii of about 0.39 nm, not much larger than a water molecule. These results show that ion specific effects are short ranged and arise primarily due to differences in the local ion-water interactions.