Comparative molecular dynamics simulations of charged solid-liquid interfaces with different water models

TL;DR

This study compares various water models in molecular dynamics simulations of charged silica-water interfaces, assessing their accuracy and consistency with continuum models, and highlights the importance of model selection for nanoscale interfacial phenomena.

Contribution

It provides a systematic comparison of water models in simulating charged solid-liquid interfaces, identifying models that align well with analytical predictions and discussing limitations at higher salt concentrations.

Findings

Certain water models show good agreement with continuum models.

Ion pairing limits simulation reproducibility at high salt concentrations.

Analytical models can be applied at the nanoscale with known Stern layer charge.

Abstract

Aqueous solid-liquid interfaces (SLI) are ubiquitous in nature and technology, often hosting molecular-level processes with macroscopic consequences. Molecular dynamics (MD) simulations offer a tool of choice to investigate interfacial phenomena with atomistic precision, but there exists a large number of water models, each optimised for a different purpose. Here we compare the ability of common water models to accurately simulate the interface between a charged silica surface and an aqueous solution containing NaCl. We first compare the bulk dielectric constant of water and its dependence on salt concentration for SPC/Fw, SPC/e, TIPS3p, H2O/DC, TIP3P-Fw, OPC3, TIP3P, TIP3P-FB, TIP3P-ST, FBA/e, and TIPS3p-PPPM, revealing large variations between models. Simulating the interface with silica for the most suitable water models (SPC/Fw, H2O/DC, TIP3-ST and TIPS3p-PPPM) show some intrinsic consistency with continuum predictions (Poisson-Boltzmann) whereby the free energy minima obtained from MD and the analytical model are in agreement, provided the latter includes the MD-determined total charge of ions in the Stern layer and dielectric constant. This consistency stands even for water models with a dielectric constant off by 100%. For salt concentrations higher than 0.21 M NaCl, the formation of random ion-ion pairs limits the reproducibility of the MD results and the applicability of the analytical method. The results highlight the applicability of the analytical model down to the nanoscale, provided a priory knowledge of the Stern layer charge is available. The findings could have significant implications for MD simulations of SLIs, especially at charged or electrified interfaces.