The interaction between colloids in polar mixtures above Tc

TL;DR

This paper investigates how colloids interact in polar mixtures near or above the critical temperature, revealing ion-specific effects and the influence of salt concentration on attraction or repulsion.

Contribution

It introduces a model showing how preferential solvent adsorption and ion solvation jointly determine colloid interactions in salt-containing mixtures near criticality.

Findings

Attractive interactions dominate far from the coexistence curve.

Ion-specific effects depend on salt amount and mixture composition.

Repulsive interactions can occur with antagonistic salts despite attractive electrostatic forces.

Abstract

We calculate the interaction potential between two colloids immersed in an aqueous mixture containing salt near or above the critical temperature. We find an attractive interaction far from the coexistence curve due to the combination of preferential solvent adsorption at the colloids' surface and preferential ion solvation. We show that the ion-specific interaction strongly depends on the amount of salt added as well as on the mixture composition. Our results are in accord with recent experiments. For a highly antagonistic salt of hydrophilic anions and hydrophobic cations, a repulsive interaction at an intermediate inter-colloid distance is predicted even though both the electrostatic and adsorption forces alone are attractive.