Range of ion specific effects in the hydration of ions

TL;DR

This paper investigates ion-specific effects in hydration, revealing that such effects are short-ranged and mainly due to local ion-water interactions, using the quasichemical approach to decompose hydration free energy.

Contribution

It introduces a detailed analysis of ion-specific effects in hydration using the quasichemical approach, highlighting the short-range nature of these effects and their dependence on local interactions.

Findings

Ion-specific effects are short-ranged.

Differences in local ion-water interactions drive ion-specific effects.

Compressive force becomes equal for similarly charged ions at about 0.39 nm radius.

Abstract

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.