A simulational and theoretical study of the spherical electrical double layer for a size-asymmetric electrolyte: the case of big coions

TL;DR

This study combines Monte Carlo simulations and theoretical models to analyze the spherical electrical double layer in size-asymmetric electrolytes, revealing the significance of ion size effects on charge distribution and screening.

Contribution

It provides a comprehensive comparison between simulation data and advanced integral equation theories, confirming the non-dominance of counterions and highlighting size asymmetry effects.

Findings

Simulations agree well with HNC/HNC and HNC/MSA theories.

Point-ion models show notable limitations.

Size asymmetry enhances charge reversal and screening.

Abstract

Monte Carlo simulations of a spherical macroion, surrounded by a size-asymmetric electrolyte in the primitive model, were performed. We considered 1:1 and 2:2 salts with a size ratio of 2 (i.e., with coions twice the size of counterions), for several surface charge densities of the macrosphere. The radial distribution functions, electrostatic potential at the Helmholtz surfaces, and integrated charge are reported. We compare these simulational data with original results obtained from the Ornstein-Zernike integral equation, supplemented by the hypernetted chain/hypernetted chain (HNC/HNC) and hypernetted chain/mean spherical approximation (HNC/MSA) closures, and with the corresponding calculations using the modified Gouy-Chapman and unequal-radius modified Gouy-Chapman theories. The HNC/HNC and HNC/MSA integral equations formalisms show good concordance with Monte Carlo "experiments", whereas the notable limitations of point-ion approaches are evidenced. Most importantly, the simulations confirm our previous theoretical predictions of the non-dominance of the counterions in the size-asymmetric spherical electrical double layer [J. Chem. Phys. 123, 034703 (2005)], the appearance of anomalous curvatures at the outer Helmholtz plane and the enhancement of charge reversal and screening at high colloidal surface charge densities due to the ionic size asymmetry.