Electrolytic depletion interactions

TL;DR

This paper introduces a theoretical framework combining depletion free-energy with Poisson-Boltzmann theory to analyze electrostatic interactions between uncharged surfaces in electrolytes, revealing distance-dependent attractive and repulsive forces.

Contribution

It develops a novel free-energy functional approach to account for interfacial selectivity effects on electrostatic interactions in electrolytes.

Findings

Interactions are always repulsive at large separations.

Attractive forces occur at shorter distances.

Transition from attraction to repulsion depends on depletion force range.

Abstract

We consider the interactions between two uncharged planar macroscopic surfaces immersed in an electrolyte solution which are induced by interfacial selectivity. These forces are taken into account by introducing a depletion free-energy density functional, in addition to the usual mean-field Poisson-Boltzmann functional. The minimization of the total free-energy functional yields the density profiles of the microions and the electrostatic potential. The disjoining pressure is obtained by differentiation of the total free energy with respect to the separation of the surfaces, holding the range and strength of the depletion forces constant. We find that the induced interaction between the two surfaces is always repulsive for sufficiently large separations, and becomes attractive at shorter separations. The nature of the induced interactions changes from attractive to repulsive at a distance corresponding to the range of the depletion forces.