Understanding the Anomalous Diffusion of Water in Aqueous Electrolytes Using Machine Learned Potentials

Abstract

The diffusivity of water in aqueous cesium iodide solutions is larger than that in neat liquid water, and vice versa for sodium chloride solutions. Such peculiar ion-specific behavior, called anomalous diffusion, is not reproduced in typical force field-based molecular dynamics (MD) simulations due to inadequate treatment of ion-water interactions. Herein, this hurdle is tackled using machine learned atomic potentials (MLPs) trained on data from density functional theory calculations. MLP-based atomistic MD simulations of aqueous salt solutions reproduce experimentally determined thermodynamic, structural, dynamical, and transport properties, including their varied trends of water diffusivities across salt concentration. This enables an examination of their intermolecular structure to unravel the microscopic underpinnings of the distinction in their transport. While both ions in CsI solutions contribute to faster diffusion of water molecules, the competition between the heavy retardation by Na-ions and slight acceleration by Cl-ions in NaCl solutions reduces their water diffusivity.

23 pages, 5 figures

References in corpus (5)

• A force field of Li${}^{+}$ , Na${}^{+}$ , K${}^{+}$, Mg${}^{2+}$, Ca${}^{2+}$, Cl${}^{-}$, and SO${}_4^{2-}$ in aqueous solution based on the TIP4P/2005 water model and scaled charges for the ions
• Polarizable Mean-Field Model of Water for Biological Simulations with Amber and Charmm force fields
• Dissolving salt is not equivalent to applying a pressure on water
• The Madrid-2019 force field for electrolytes in water using TIP4P/2005 and scaled charges: extension to the ions F${}^{-}$, Br${}^{-}$, I${}^{-}$, Rb${}^{+}$, Cs${}^{+}$
• Scaled charges for ions: an improvement but not the final word for modeling electrolytes in water