Primitive Model Electrolytes in the Near and Far Field: Decay Lengths from DFT and Simulations

TL;DR

This study investigates decay lengths in primitive electrolyte models using DFT, IET, and MD simulations, finding that primitive models cannot explain the large decay lengths observed experimentally in concentrated electrolytes.

Contribution

The paper compares decay lengths from DFT, IET, and MD for primitive electrolyte models, revealing limitations of these models in explaining experimental observations.

Findings

Decay lengths from primitive models match in the far field for some approaches.

Primitive models do not reproduce the large decay lengths seen in experiments.

MD simulations support the DFT and IET results, highlighting model limitations.

Abstract

Inspired by recent experimental observations of anomalously large decay lengths in concentrated electrolytes, we revisit the Restricted Primitive Model (RPM) for an aqueous electrolyte. We investigate the asymptotic decay lengths of the one-body ionic density profiles for the RPM in contact with a planar electrode using classical Density Functional Theory (DFT), and compare these with the decay lengths of the corresponding two-body correlation functions in bulk systems, obtained in previous Integral Equation Theory (IET) studies. Extensive Molecular Dynamics (MD) simulations are employed to complement the DFT and IET predictions. Our DFT calculations incorporate electrostatic interactions between the ions using three different (existing) approaches: one based on the simplest mean field treatment of Coulomb interactions (MFC), whilst the other two employ the Mean Spherical approximation (MSA). The MSAc invokes only the MSA bulk direct correlation function whereas the MSAu also incorporates the MSA bulk internal energy. Although MSAu yields profiles that agree best with MD simulations in the near field, in the far field we observe that the decay lengths are consistent between IET, MSAc, and MD simulations, whereas those from MFC and MSAu deviate significantly. Using DFT we calculated the solvation force, which relates directly to surface force experiments. We find that its decay length is neither qualitatively nor quantitatively close to the large decay lengths measured in experiments and conclude that the latter cannot be accounted for by the primitive model. The anomalously large decay lengths found in surface force measurements require an explanation that lies beyond primitive models.