Attraction between Neutral Dielectrics Mediated by Multivalent Ions in an Asymmetric Ionic Fluid

TL;DR

This study investigates how multivalent ions in an asymmetric ionic fluid induce attractive forces between neutral dielectric surfaces, considering image charge effects, ion confinement, and correlations through simulations and analytical theory.

Contribution

It introduces an analytical theory incorporating ion-image charge correlations and validates it against simulations, revealing significant attraction effects at the nanoscale.

Findings

Attractive interactions are comparable to van der Waals forces.

Implicit-ion models match explicit-ion simulations at high salt concentrations.

Ion correlations significantly influence surface interactions.

Abstract

We study the interaction between two neutral plane-parallel dielectric bodies in the presence of a highly asymmetric ionic fluid, containing multivalent as well as monovalent (salt) ions. Image charge interactions, due to dielectric discontinuities at the boundaries, as well as effects from ion confinement in the slit region between the surfaces are taken fully into account, leading to image-generated depletion attraction, ion correlation attraction and steric-like repulsive interactions. We investigate these effects by employing a combination of methods including explicit-ion and implicit-ion Monte-Carlo simulations, as well as an effective interaction potential analytical theory. The latter incorporates strong ion-image charge correlations, which develop in the presence of high valency ions in the mixture. We show that the implicit-ion simulations and the proposed analytical theory can describe the explicit simulation results on a qualitative level, while excellent quantitative agreement can be obtained for sufficiently large monovalent salt concentrations. The resultant attractive interaction between the neutral surfaces is shown to be significant, as compared with the usual van der Waals interactions between semi-infinite dielectrics, and can thus play a significant role at the nano scale.