In-plane structure of the electric double layer in the primitive model using classical density functional theory

TL;DR

This study uses classical density functional theory to accurately predict the in-plane structure of electric double layers in the primitive model, revealing that observed structural transitions are likely due to specific ion interactions rather than fundamental effects.

Contribution

It demonstrates that an appropriate DFT functional can match MD simulation results for EDL in the primitive model, clarifying the nature of in-plane structural transitions.

Findings

DFT accurately predicts in-plane EDL structure in the primitive model.

No evidence of a fundamental structural crossover in the EDL.

Previously observed transitions are attributed to specific ion force fields.

Abstract

The electric double layer (EDL) has a pivotal role in screening charges on surfaces as in supercapacitor electrodes or colloidal and polymer solutions. Its structure is determined by correlations between the finite-sized ionic charge carriers of the underlying electrolyte and, this way, these correlations affect the properties of the EDL and of applications utilizing EDLs. We study the structure of EDLs within classical density functional theory (DFT) in order to uncover whether a structural transition in the first layer of the EDL that is driven by changes in the surface potential depends on specific particle interactions or has a general footing. This transition has been found in full-atom simulations. Thus far, investigating the in-plane structure of the EDL for the primitive model (PM) using DFT proved a challenge. We show here that the use of an appropriate functional predicts the in-plane structure of EDLs in excellent agreement with molecular dynamics (MD) simulations. This provides the playground to investigate how the structure factor within a layer parallel to a charged surface changes as function of both the applied surface potential and its separation from the surface. We discuss pitfalls in properly defining an in-plane structure factor and fully map out the structure of the EDL within the PM for a wide range of electrostatic electrode potentials. However, we do not find any signature of a structural crossover and conclude that the previously reported effect is not fundamental but rather occurs due to the specific force field of ions used in the simulations.