Interactions between conducting surfaces in salt solutions.

Samuel Stenberg,Clifford E Woodward,Jan Forsman

doi:10.1039/d1sm01520f

Abstract

In this work, we simulate interactions between two perfectly conducting surfaces, immersed in a salt solution. We demonstrate that these forces are quantitatively different from those between (equally charged) non-conducting surfaces. There is, for instance, a significant repulsion between net neutral surfaces. On the other hand, there are also qualitative similarities, with behaviours found with non-conducting surfaces. For instance, there is a non-monotonic dependence of the free energy barrier height, on the salt concentration, and the minimum essentially coincides with a flat profile of the apparent surface charge density (i.e. the effective net surface charge density, some distance away from the surface, when accounting for ion neutralization), outside the so-called Stern layer. These conditions can be described as "perfect surface charge neutralization". Despite observed quantitative differences, we demonstrate that it might be possible to mimic a dispersion containing charged colloidal metal particles by a simpler model system with charged non-conducting particles, using modified particle-ion interactions.

Highlights

Interactions between particles and charged surfaces in aqueous salt solutions are important and ubiquitous in many areas of soft matter
We will start by investigating the response of surface forces to a change of Cbias, which in turn leads to different surface charge densities
Given that our intent in this work is to establish possible relations between apparent surface charge density profiles and free energy barriers, we will focus on long-range behaviours

Summary

Introduction

Interactions between particles and charged surfaces in aqueous salt solutions are important and ubiquitous in many areas of soft matter. At high electrostatic coupling strengths, e.g., in the presence of multivalent ions, numerous theoretical and experimental studies have illustrated that a simple Poisson–Boltzmann (PB) treatment, even at the non-linear level, may lead to predicted behaviours that are qualitatively wrong. The origin of this failure is the mean-field approximation upon which the PB theory is based. It would be of interest to devise simplified approaches, wherein the computationally costly effects of surface polarization can be approximately taken into account This would be especially useful when dealing with dispersions containing colloidal conducting particles, wherein multiple image interactions are exceptionally difficult to handle. We introduce a simple effective model that mimics the conducting case by mapping it onto a model of a charged non-conducting particle with an additional (suitably chosen) particle–ion interaction

Model and methods

Results

Conclusions