Structure Formation due to Antagonistic Salts

TL;DR

This paper explores how antagonistic salts induce structure formation in water-oil mixtures through charge and composition coupling, leading to mesophases and oscillatory disturbances near criticality, supported by theoretical calculations.

Contribution

It introduces a theoretical framework for understanding structure formation driven by antagonistic salts in solvent mixtures, highlighting mechanisms at interfaces and near critical points.

Findings

Formation of electric double layers reduces surface tension.

Mesophases with periodic structures are predicted and match experimental observations.

Oscillatory disturbances occur near solid surfaces at criticality.

Abstract

Antagonistic salts are composed of hydrophilic and hydrophobic ions. In a mixture solvent (water-oil) such ion pairs are preferentially attracted to water or oil, giving rise to a coupling between the charge density and the composition. First, they form a large electric double layer at a water-oil interface, reducing the surface tension and producing mesophases. Here, the cations and anions are loosely bound by the Coulomb attraction across the interface on the scale of the Debye screening length. Second, on solid surfaces, hydrophilic (hydrophobic) ions are trapped in a water-rich (oil-rich) adsorption layer, while those of the other species are expelled from the layer. This yields a solvation mechanism of local charge separation near a solid. In particular, near the solvent criticality, disturbances around solid surfaces can become oscillatory in space. In mesophases, we calculate periodic structures, which resemble those in experiments.