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

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, and ±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 to 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 for 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.

Computation of Electrical Conductivities of Aqueous Electrolyte Solutions: Two Surfaces, One Property

“…This is not entirely surprising, as the Madrid-2019 force field (that uses a scaled charge of ±0.85) improves the description of transport properties of electrolytes in water but is not able to yield quantitative agreement with experiments. To this end, we have recently proposed a force field for NaCl and KCl (denoted as Madrid-Transport) that is able to predict transport properties of these electrolyte solutions with excellent agreement to the experiment. , Therefore, we shall compute the electrical conductivities of NaCl and KCl solutions using the Madrid-Transport force field (which uses a scaled charge of ±0.75). We shall implement the main idea of this work, namely, to use scaled charges to obtain the trajectories and integer charges to describe the DMS (i.e., using integer charges in eqs –).…”

“…As we have recently shown, the scaled charge can be taken as a fitting parameter depending on the property that one wants to reproduce. 61 Transport properties are among the most interesting properties that can be studied by simulation and that traditional force fields of electrolytes have never been able to reproduce correctly. 89 In our recent work, we proposed the Madrid-Transport force field, 60,61 which uses a scaled charge of q = ±0.75 and that is able to reproduce transport properties, such as the viscosities and diffusion coefficients of water and ions in the whole concentration range.…”

“…Transport properties are among the most interesting properties that can be studied by simulation and that traditional force fields of electrolytes have never been able to reproduce correctly . In our recent work, we proposed the Madrid-Transport force field, , which uses a scaled charge of q = ±0.75 and that is able to reproduce transport properties, such as the viscosities and diffusion coefficients of water and ions in the whole concentration range. This force field has also been able to reproduce transport properties in the presence of hydrogen .…”

Thermodynamic and Transport Properties of H2/H2O/NaB(OH)4 Mixtures Using the Delft Force Field (DFF/B(OH)4–)

“…This water force field can accurately capture the densities, self-diffusivities, and viscosities of pure H 2 O and H 2 O solutions (e.g., aqueous NaCl, , KCl, NaOH, and KOH) for a wide range of conditions. ,− For Na + ions, the Madrid-2019 and Madrid-Transport force fields are used. Both Madrid Na + force fields are non-polarizable force fields, which make use of scaled charges. , In non-polarizable models, charges of ions in water and for ionic liquids are commonly scaled down. ,, In the Madrid-2019 and Madrid-Transport model, the charges of Na + ions are scaled down to 0.85 and 0.75, respectively. This charge scaling leads to more accurate viscosity predictions for aqueous electrolyte solutions (e.g., NaCl and Na 2 SO 4 ) compared to using unscaled charges .…”

“…This charge scaling leads to more accurate viscosity predictions for aqueous electrolyte solutions (e.g., NaCl and Na 2 SO 4 ) compared to using unscaled charges . The use of scaled charges is discussed in detail in refs , , . The three-site Marx model is used for computing the H 2 solubilities and self-diffusion coefficients.…”