Modeling the Behavior of Complex Aqueous Electrolytes Using Machine Learning Interatomic Potentials: The Case of Sodium Sulfate

TL;DR

This paper develops a machine learning interatomic potential trained on DFT data to accurately simulate the structure and thermodynamics of complex aqueous electrolytes, specifically sodium sulfate, beyond the limitations of traditional methods.

Contribution

The study introduces a MLIP approach that reproduces bulk properties and hydration structures of sodium sulfate, enabling long-timescale simulations at DFT accuracy.

Findings

MLIP accurately reproduces density and radial distribution functions.

Sulfate ions are strongly solvated at low concentrations, stabilizing solvent-separated pairs.

Ion pairing involves a multistep coordination process.

Abstract

Understanding the structure and thermodynamics of solvated ions is essential for advancing applications in electrochemistry, water treatment, and energy storage. While ab initio molecular dynamics methods are highly accurate, they are limited by short accessible time and length scales whereas classical force fields struggle with accuracy. Herein, we explore the structure and thermodynamics of complex monovalent-divalent ion pairs using Na2SO4(aq) as a case study by applying a machine learning interatomic potential (MLIP) trained on density functional theory (DFT) data. Our MLIP-based approach reproduces key bulk properties such as density and radial distribution functions of water. We provide the hydration structure of the sodium and sulfate ions in 0.1-2 M concentration range and the one-dimensional and two-dimensional potentials of mean force for the sodium-sulfate ion pairing at the low concentration limit (0.1 M) which are inaccessible to DFT. At low concentrations, the sulfate ion is strongly solvated, leading to the stabilization of solvent-separated ion pairs over contact ion pairs. Minimum energy pathway analysis revealed that coordinating two sodium ions with a sulfate ion is a multistep process whereby the sodium ions coordinate to the sulfate ion sequentially. We demonstrate that MLIPs allow the study of solvated ions beyond simple monovalent pairs with DFT-level accuracy in their low concentration limit (0.1 M) via statistically converged properties from ns long simulations.