Effects of image charges, interfacial charge discreteness, and surface roughness on the zeta potential of spherical electric double layers

TL;DR

This study uses Monte Carlo simulations to analyze how image charges, charge discreteness, and surface roughness influence the zeta potential and electric double layer structure around spherical particles in electrolyte solutions.

Contribution

It systematically compares different models of interfacial charges and highlights the significant effects of image charges and excluded volumes on charge inversion phenomena.

Findings

Image charges enhance charge inversion with monovalent charges.

Excluded volume effects modify charge inversion depending on ion valency.

Potential at one ion diameter from the surface is suitable for defining zeta potential.

Abstract

We investigate the effects of image charges, interfacial charge discreteness, and surface roughness on spherical electric double layers in electrolyte solutions with divalent counter-ions in the setting of the primitive model. By using Monte Carlo simulations and the image charge method, the zeta potential profile and the integrated charge distribution function are computed for varying surface charge strengths and salt concentrations. Systematic comparisons were carried out between three distinct models for interfacial charges: 1) SURF1 with uniform surface charges, 2) SURF2 with discrete point charges on the interface, and 3) SURF3 with discrete interfacial charges and finite excluded volume. By comparing the integrated charge distribution function (ICDF) and potential profile, we argue that the potential at the distance of one ion diameter from the macroion surface is a suitable location to define the zeta potential. In SURF2 model, we find that image charge effects strongly enhance charge inversion for monovalent interfacial charges, and strongly suppress charge inversion for multivalent interfacial charges. For SURF3, the image charge effect becomes much smaller. Finally, with image charges in action, we find that excluded volumes (in SURF3) suppress charge inversion for monovalent interfacial charges and enhance charge inversion for multivalent interfacial charges. Overall, our results demonstrate that all these aspects, i.e., image charges, interfacial charge discreteness, their excluding volumes have significant impacts on the zeta potential, and thus the structure of electric double layers.