Surface Tension of Electrolyte Interfaces: Ionic Specificity within a Field-Theory Approach

TL;DR

This paper develops a field-theory model to accurately predict the surface tension of ionic solutions at interfaces, incorporating ionic specificity and extending beyond mean-field approximations, aligning well with experimental data.

Contribution

It introduces a first-order loop expansion approach that unifies classical and ionic-specific effects on surface tension, improving theoretical predictions.

Findings

Analytical formulas for surface tension dependence on ionic parameters.

The model fits experimental data across various salt concentrations.

Reproduces the reverse Hofmeister series for ions at interfaces.

Abstract

We study the surface tension of ionic solutions at air/water and oil/water interfaces. By using field-theoretical methods and including a finite proximal surface-region with ionic-specific interactions. The free energy is expanded to first-order in a loop expansion beyond the mean-field result. We calculate the excess surface tension and obtain analytical predictions that reunite the Onsager-Samaras pioneering result (which does not agree with experimental data), with the ionic specificity of the Hofmeister series. We derive analytically the surface-tension dependence on the ionic strength, ionic size and ion-surface interaction, and show consequently that the Onsager-Samaras result is consistent with the one-loop correction beyond the mean-field result. Our theory fits well a wide range of salt concentrations for different monovalent ions using one fit parameter per electrolyte, and reproduces the reverse Hofmeister series for anions at the air/water and oil/water interfaces.