Dielectric decrement as a source of ion specific effects

TL;DR

This paper introduces a mean-field model highlighting how local dielectric constant variations caused by ions contribute to ion-specific effects, influencing surface tension and force measurements.

Contribution

It proposes a novel source of ion-specificity based on dielectric decrement, expanding understanding beyond traditional dispersion and surface-ion interaction mechanisms.

Findings

Depletion of ions near charged surfaces due to dielectric decrement.

Increase in pressure between charged plates caused by dielectric heterogeneity.

Dielectric effects should be considered in interpreting ion-specific experimental results.

Abstract

Many theoretical studies were devoted in the past to ion-specific effects, trying to interpret a large body of experimental evidence, such as surface tension at air/water interfaces and force measurements between charged objects. Although several mechanisms were suggested to explain the results, such as dispersion forces and specific surface-ion interactions, we would like to suggest another source of ion-specificity, originating from the local variations of the dielectric constant due to the presence of ions in the solution. We present a mean-field model to account for the heterogeneity of the dielectric constant caused by the ions. In particular, for ions that decrease the dielectric constant we find a depletion of ions from the vicinity of charged surfaces. For a two-plate system, the same effect leads to an increase of the pressure in between two surfaces. Our results suggest that the effect of ions on the local dielectric constant should be taken into account when interpreting experiments that address ion-specific effects.