How ions in solution can change the sign of the critical Casimir potential

TL;DR

This paper demonstrates how ions in a near-critical aqueous mixture can alter the sign of the critical Casimir potential between surfaces, leading to attraction or repulsion depending on surface properties and ion effects.

Contribution

It introduces a microscopic theory combining Landau and Debye-Hückel theories to explain ion-induced sign changes in the critical Casimir potential.

Findings

Hydrophilic ions can cause attraction between like-charge surfaces with opposing adsorption.

Oppositely charged hydrophobic surfaces can repel each other due to ion effects.

The sign change depends on the screening length and surface selectivity.

Abstract

We show that hydrophilic ions present in a confined, near-critical aqueous mixture can lead to an attraction between like charge surfaces with opposing preferential adsorption of the two species of the mixture, even though the corresponding Casimir potential in uncharged systems is repulsive. This prediction agrees with recent experiment [Nellen {\it{et al.}}, Soft Matter{\bf{80}}, 061143 (2011)]. We also show that oppositely charged hydrophobic surfaces can repel each other, although the Casimir potential between uncharged surfaces with like preferential adsorption (selectivity) is attractive. This behavior is expected when the electrostatic screening length is larger than the correlation length, and one of the confining surfaces is strongly selective and weakly charged, whereas the other confining surface is weakly selective and strongly charged. The Casimir potential can change sign because the hydrophilic ions near the weakly hydrophobic surface can overcompensate the effect of hydrophobicity, and this surface can act as a hydrophilic one. We also predict a more attractive interaction between hydrophilic surfaces and a more repulsive interaction between hydrophobic surfaces than given by the sum of the Casimir and Deby-H\"uckel potentials. Our theory is derived systematically from a microscopic approach, and combines the Landau-type and Debye-H\"uckel theories with an additional contribution of an entropic origin.