Electrolytes in regimes of strong confinements: surface charge modulations, osmotic equilibrium and electroneutrality

TL;DR

This study investigates electrolytes confined between charged surfaces, exploring how surface charge modulations and different reservoir models affect ionic distributions and surface interactions, especially under strong confinement where local electroneutrality may not hold.

Contribution

It introduces an improved Monte Carlo simulation and a linear Debye-Hückel-based approach to analyze electrolyte behavior in strongly confined geometries with modulated surface charges, highlighting the importance of reservoir modeling.

Findings

Surface interactions strongly depend on the reservoir model used.

Local electroneutrality may be violated in strong confinement.

Choice of ionic exchange scenario significantly affects surface forces.

Abstract

In the present work, we study an electrolyte solution confined between planar surfaces with nonopatterned charged domains, which has been connected to a bulk ionic reservoir. The system is investigated through an improved Monte Carlo (MC) simulation method, suitable for simulation of electrolytes in the presence of modulated surface charge distributions. We also employ a linear approach in the spirit of the classical Debye-H\"uckel approximation, which allows one to obtain explicit expressions for the averaged potentials, ionic profiles, effective surface interactions and the net ionic charge confined between the walls. Emphasis is placed in the limit of strongly confined electrolytes, in which case local electroneutrality in the inter-surface space might not be fulfilled. In order to access the effects of such lack of local charge neutrality on the ionic-induced interactions between surfaces with modulated charge domains, we consider two distinct model systems for the confined electrolyte: one in which a salt reservoir is explicitly taken into account {\it via} the osmotic equilibrium with an electrolyte of fixed bulk concentration, and a second one in which the equilibrium with a charge neutral ionic reservoir is implicitly considered. While in the former case the osmotic ionic exchange might lead to non-vanishing net charges, in the latter model charge neutrality is enforced through the appearance of an implicit Donnan potential across the charged interfaces. A strong dependence of the ionic-induced surface interactions in the employed model system is observed at all particle separations. These findings strongly suggest that due care is to be taken while choosing among different scenarios to describe the ionic exchanging in electrolytes confined between charged surfaces, even in cases when the monopole (non zero net charge) surface contributions are absent.