Influence of solvent polarization and non-uniform ion size on electrostatic properties between charged surfaces in an electrolyte solution

TL;DR

This study models electrostatic interactions between charged surfaces in an electrolyte, considering solvent polarization and non-uniform ion sizes, revealing how these factors influence electrostatic potential and osmotic pressure.

Contribution

It introduces a mean-field framework that accounts for solvent polarization and unequal ion sizes, providing new insights into electrostatic properties between charged surfaces.

Findings

Electrostatic potential increases with counterion size for similar charges.

Asymmetry in properties occurs for oppositely charged surfaces due to ion size effects.

Solvent polarization decreases electrostatic potential and osmotic pressure.

Abstract

In this paper, we study electrostatic properties between two similar or oppositely charged surfaces immersed in an electrolyte solution by using mean-field approach accounting for solvent polarization and non-uniform size effect. Applying a free energy formalism accounting for unequal ion sizes and orientational ordering of water dipoles, we derive coupled and self-consistent equations to calculate electrostatic properties between charged surfaces. Electrostatic properties for similarly charged surfaces depend on counterion size but not coion size. Moreover, electrostatic potential and osmotic pressure between similarly charged surfaces are found to be increased with increasing counterion size. On the other hand, the corresponding ones between oppositely charged surfaces are related to both sizes of positive and negative ions. For oppositely charged surfaces, the electrostatic potential, number density of solvent molecules and relative permittivity of an electrolyte having unequal ion sizes are not symmetric about the centerline between the charged surfaces. For either case, the consideration of solvent polarization results in an decrease in the electrostatic potential and the osmotic pressure compared to the case without the effect.