Surface tension of electrolytes: Hydrophilic and hydrophobic ions near an interface

TL;DR

This paper models ion distributions at fluid interfaces considering solvation and image interactions, revealing how hydrophilic and hydrophobic ions influence surface tension and induce microphase separation, supported by x-ray reflectivity data.

Contribution

It provides a general expression for electrolyte surface tension incorporating electrostatic effects and explains ion distribution behaviors at interfaces with new theoretical insights.

Findings

Hydrophilic and hydrophobic ions cause microphase separation at interfaces.

Surface tension includes a negative electrostatic term proportional to the square root of salt density.

Ion distributions match x-ray reflectivity measurements for mixed ion systems.

Abstract

We calculate the ion distributions around an interface in fluid mixtures of highly polar and less polar fluids (water and oil) for two and three ion species. We take into account the solvation and image interactions between ions and solvent. We show that hydrophilic and hydrophobic ions tend to undergo a microphase separation at an interface, giving rise to an enlarged electric double layer. We also derive a general expression for the surface tension of electrolyte systems, which contains a negative electrostatic contribution proportional to the square root of the bulk salt density. The amplitude of this square-root term is small for hydrophilic ion pairs, but is much increased for hydrophilic and hydrophobic ion pairs. For three ion species including hydrophilic and hydrophobic ions, we calculate the ion distributions to explain those obtained by x-ray reflectivity measurements.