Cluster Formation induced by local dielectric saturation in Restricted Primitive Model Electrolytes

TL;DR

This study introduces a modified electrolyte model incorporating local dielectric saturation effects, revealing that ion clustering and long-range interactions observed experimentally are due to dielectric heterogeneity, not just Coulomb enhancement.

Contribution

The paper presents a novel modification to the Restricted Primitive Model by including local dielectric saturation, explaining experimentally observed ion clustering and long-range surface interactions.

Findings

Model exhibits strong ion clustering in simulations.

Clusters lead to long-range interactions similar to experiments.

Uniform dielectric models do not produce similar clustering.

Abstract

Experiments using the Surface Force Apparatus (SFA) have found anomalously long ranged charge-charge underscreening in concentrated salt solutions. Meanwhile, theory and simulations have suggested ion clustering to be the possible origin of this behaviour. The popular Restricted Primitive Model of electrolyte solutions, in which the solvent is represented by a uniform relative dielectric constant, $\varepsilon_r$, is unable to resolve the anomalous underscreening seen in experiments. In this work, we modify the Restricted Primitive Model to account for local dielectric saturation within the ion hydration shell. The dielectric constant in our model locally decreases from the bulk value to a lower saturated value at the ionic surface. The parameters for the model are deduced so that typical salt solubilities are obtained. Our simulations for both bulk and slit geometries show that our model displays strong cluster formation and these give rise to long-ranged interactions between charged surfaces at distances similar to what has been observed in SFA measurements. An electrolyte model wherein the dielectric constant remains uniform does not display similar clusters, even with $\varepsilon_r$ equal to the saturated value at ion contact. Hence, the observed behaviours are not simply due to an enhanced Coulomb interaction.