Liquid-liquid interfacial tension of electrolyte solutions

TL;DR

This paper theoretically analyzes how the excess liquid-liquid interfacial tension in electrolyte solutions varies with ionic strength, revealing asymptotic behaviors and crossover regimes influenced by material parameters.

Contribution

It introduces a theoretical framework describing the asymptotic dependence of interfacial tension on ionic strength and identifies crossover behaviors based on material properties.

Findings

Interfacial tension scales as O(- I^0.5) at low ionic strength.

Interfacial tension scales as O(+- I) at high ionic strength.

In ion partitioning-free systems, the surface tension follows an O(- I ln(I)) law.

Abstract

It is theoretically shown that the excess liquid-liquid interfacial tension between two electrolyte solutions as a function of the ionic strength I behaves asymptotically as O(- I^0.5) for small I and as O(+- I) for large I. The former regime is dominated by the electrostatic potential due to an unequal partitioning of ions between the two liquids whereas the latter regime is related to a finite interfacial thickness. The crossover between the two asymptotic regimes depends sensitively on material parameters suggesting that, depending on the actual system under investigation, the experimentally accessible range of ionic strengths can correspond to either the small or the large ionic strength regime. In the limiting case of a liquid-gas surface where ion partitioning is absent, the image chage interaction can dominate the surface tension for small ionic strength I such that an Onsager-Samaras limiting law O(- I ln(I)) is expected. The proposed picture is consistent with more elaborate models and published measurements.