Surface tension between liquids containing antagonistic and regular salts

We investigate theoretically the surface tension between two immiscible liquids containing antagonistic and regular salts. The mean-field model includes the electrostatic energy and the Gibbs transfer energy of the ions. We find the Donnan potential difference between the liquids and solve the Poisson–Boltzmann equation to calculate the potential and ion density profiles. When the liquids contain only a regular salt, the surface tension increases when compared to the no-salt case; in contrast, the surface tension is reduced when they contain only an antagonistic salt. When both salts are present, the surface tension can either further decrease or increase depending on the preferential solvation of the ions, in agreement with published experimental observations.