Strong coupling electrostatics for randomly charged surfaces: Antifragility and effective interactions

TL;DR

This paper investigates how strong electrostatic coupling and surface charge disorder influence effective interactions between dielectric surfaces, revealing antifragility and significant disorder-induced attractions surpassing van der Waals forces.

Contribution

It introduces a detailed analysis of the interplay between charge disorder, dielectric effects, and multivalent ions, uncovering antifragility and non-monotonic interactions in strongly coupled Coulomb fluids.

Findings

Disorder induces a singular counterion density profile at surfaces.

Effective attraction can be stronger than van der Waals forces.

System exhibits antifragility, reducing thermal disorder through strong coupling effects.

Abstract

We study the effective interaction mediated by strongly coupled Coulomb fluids between dielectric surfaces carrying quenched, random monopolar charges with equal mean and variance, both when the Coulomb fluid consists only of mobile multivalent counterions and when it consists of an asymmetric ionic mixture containing multivalent and monovalent (salt) ions in equilibrium with an aqueous bulk reservoir. We analyze the consequences that follow from the interplay between surface charge disorder, dielectric and salt image effects, and the strong electrostatic coupling that results from multivalent counterions on the distribution of these ions and the effective interaction pressure they mediate between the surfaces. In a dielectrically homogeneous system, we show that the multivalent counterions are attracted towards the surfaces with a singular, disorder-induced potential that diverges logarithmically on approach to the surfaces, creating a singular counterion density profile with an algebraic divergence at the surfaces. This effect drives the system towards a state of lower thermal "disorder", one that can be described by a renormalized temperature, exhibiting thus a remarkable antifragility. The interaction pressure acting on the surfaces displays in general a highly non-monotonic behavior as a function of the inter-surface separation with a prominent regime of attraction at small to intermediate separations. This attraction is caused directly by the combined effects from charge disorder and strong coupling electrostatics of multivalent counterions, which can be quite significant even with a small degree of surface charge disorder relative to the mean surface charge. The strong coupling, disorder-induced attraction is typically far more stronger than the van der Waals interaction between the surfaces, especially within a range of several nanometers for the inter-surface separation.