Role of Water Model on Ion Dissociation at Ambient Conditions

TL;DR

This study investigates how different water models influence ion pair dissociation and stability in water, highlighting the sensitivity of ion pairing to model parameters and water polarizability effects.

Contribution

It demonstrates that the choice of water model significantly affects ion pair stability predictions, emphasizing the role of dielectric properties and polarizability in simulations.

Findings

Classical water models yield similar contact ion pair stability estimates.

Density functional theory models show greater variability in ion pair stability.

Small changes in solution density can alter the equilibrium between ion pair configurations.

Abstract

We study ion pair dissociation in water at ambient conditions using a combination of classical and ab initio approaches. The goal of this study is to disentangle the sources of discrepancy observed in computed potentials of mean force. In particular we aim to understand why some models favor the stability of solvent-separated ion pairs versus contact ion pairs. We found that some observed differences can be explained by non-converged simulation parameters. However, we also unveil that for some models, small changes in the solution density can have significant effects on modifying the equilibrium balance between the two configurations. We conclude that the thermodynamic stability of contact and solvent-separated ion pairs is very sensitive to the dielectric properties of the underlying simulation model. In general, classical models are very robust on providing a similar estimation of the contact ion pair stability, while this is much more variable in density functional theory-based models. The barrier to transition from solvent-separated to contact ion pair is fundamentally dependent on the balance between electrostatic potential energy and entropy. This reflects the importance of water intra- and inter-molecular polarizability in obtaining an accurate description of the screened ion-ion interactions.