Scaled charges for ions: an improvement but not the final word for modeling electrolytes in water

TL;DR

This paper evaluates scaled charge force fields for NaCl in water, showing that different charges optimize different properties, but no single value captures all experimental data perfectly.

Contribution

It systematically assesses multiple scaled charge models for NaCl in water, highlighting their strengths and limitations for various physical properties.

Findings

Charges ≥ 0.85 best describe structural properties

Charge 0.92 reproduces surface tension accurately

Charge 0.75 accurately models viscosities and diffusion

Abstract

In this work we discuss the use of scaled charges when developing force fields for NaCl in water. We shall develop force fields for Na$^+$ and Cl$^-$ using the following values for the scaled charge (in electron units) : 0.75, 0.80, 0.85, 0.92 along with the TIP4P/2005 model of water (for which previous force fields were proposed for q = 0.85 and q = 1). The properties considered in this work are: densities, structural properties, transport properties, surface tension, freezing point depression and maximum in density. All the developed models were able to describe quite well the experimental values of the densities. Structural properties were well described by models with charges equal or larger than 0.85, surface tension by the charge 0.92, maximum in density by the charge 0.85 and transport properties by the charge 0.75. The use of a scaled charge of 0.75 is able to reproduce with high accuracy the viscosities and diffusion coefficients of NaCl solutions by the first time. We have also considered the case of KCl in water and the results obtained were fully consistent with those of NaCl. There is no value of the scaled charge able to reproduce all the properties considered in this work. Although certainly scaled charges are not the final word in the development of force fields for electrolytes in water its use may have some practical advantages. Certain values of the scaled charge could be the best option when the interest is to describe certain experimental properties.