Charge reversal and surface charge amplification in asymmetric valence restricted primitive model planar electric double layers in the modified Poisson-Boltzmann theory

TL;DR

This paper revisits the modified Poisson-Boltzmann theory to analyze charge reversal and surface charge amplification in asymmetric electrolytes, comparing theoretical predictions with Monte Carlo simulations for improved understanding of electric double layers.

Contribution

It introduces a new formulation of the MPB theory and compares its accuracy to previous versions in modeling asymmetric electrolyte double layers.

Findings

The new theory predicts charge reversal phenomena with accuracy comparable to MPB5.

Surface charge amplification is effectively modeled in 3:1/1:3 systems.

Some artifacts appear at low concentrations, indicating limits of the theory.

Abstract

The modified Poisson-Boltzmann theory of the restricted primitive model double layer is revisited and recast in a fresh, slightly broader perspective. Derivation of relevant equations follow the techniques utilized in the earlier MPB4 and MPB5 formulations and clarifies the relationship between these. The MPB4, MPB5, and a new formulation of the theory are employed in an analysis of the structure and charge reversal phenomenon in asymmetric 2:1/1:2 valence electrolytes. Furthermore, polarization induced surface charge amplification is studied in 3:1/1:3 systems. The results are compared to the corresponding Monte Carlo simulations. The theories are seen to predict the "exact" simulation data to varying degrees of accuracy ranging from qualitative to almost quantitative. The results from a new version of the theory are found to be of comparable accuracy as the MPB5 results in many situations. However, in some cases involving low electrolyte concentrations, theoretical artifacts in the form of un-physical "shoulders" in the singlet ionic distribution functions are observed.