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

Abstract

In this work, we computed electrical conductivities under ambient conditions of aqueous NaCl and KCl solutions by using the Einstein−Helfand equation. Common force fields (charge q = ±1 e) do not reproduce the experimental values of electrical conductivities, viscosities, and diffusion coefficients. Recently, we proposed the idea of using different charges to describe the potential energy surface (PES) and the dipole moment surface (DMS). In this work, we implement this concept. The equilibrium trajectories required to evaluate electrical conductivities (within linear response theory) were obtained by using scaled charges (with the value q = ±0.75 e) to describe the PES. The potential parameters were those of the Madrid-Transport force field, which accurately describe viscosities and diffusion coefficients of these ionic solutions. However, integer charges were used to compute the conductivities (thus describing the DMS). The basic idea is that although the scaled charge describes the ion−water interaction better, the integer charge reflects the value of the charge that is transported due to the electric field. The agreement obtained with experiments is excellent, as for the first time electrical conductivities (and the other transport properties) of NaCl and KCl electrolyte solutions are described with high accuracy for the whole concentration range up to their solubility limit. Finally, we propose an easy way to obtain a rough estimate of the actual electrical conductivity of the potential model under consideration using the approximate Nernst−Einstein equation, which neglects correlations between different ions.

Molecular Dynamics Simulation on the Charge Transport Properties in a Salt-in-Ionic Liquid Electrolyte

“…It is worth mentioning that a charge scaling scheme , was adopted to account for the electronic polarization effect in the mean-field manner, with charges of all atoms were scaled by 0.7. Nonetheless, the scaled charges were only applied in the MD simulations, while the unscaled charges were adopted in calculating the conductivity spectrum because it depends on the dipole moment surface rather than the potential energy surface …”

Accurate Free Energies of Aqueous Electrolyte Solutions from Molecular Simulations with Non-polarizable Force Fields

“…Accurate modeling of vapor–liquid equilibria (VLE) of aqueous electrolyte systems is crucial for a variety of applications, such as wastewater treatment, water electrolysis, − and biomedical applications. , Modeling aqueous electrolytes is a significant challenge as a result of long-range electrostatic interactions that make solutions highly non-ideal. , Significant efforts are made to develop analytical models [equations of state (EOS)] for aqueous electrolytes. − Although these models are computationally efficient, they rely on existing thermophysical data for parametrization and do not offer atomistic insight. , Molecular simulation is a powerful tool for atomistic modeling and predicting thermodynamic and transport properties of aqueous electrolyte solutions at different temperatures, pressures, and electrolyte concentrations. − The accuracy of molecular simulations depends upon the potential energy surface (PES) that is used to compute the interactions between different species. − The PES of aqueous electrolyte solutions can be computed from ab initio calculations or semi-empirical force fields. ,, …”

“…On the basis of TIP4P/2005 water, different force fields for salts (e.g., NaCl, KCl, and KOH) have been developed. ,,− The charges of ion force fields are commonly scaled down (usually by a factor of 0.85 or 0.75 , ) , to account for the effective charge screening that occurs in the aqueous medium. , Charge scaling follows from the Electronic Continuum Correction and accounts for polarizability of ions in a mean-field way. , Using the “scaled charge” force fields of Madrid-2019 (scaled charges of +0.85/–0.85), Madrid-Transport (scaled charges of +0.75/–0.75), and the Delft Force Field of OH – (DFF/OH – ) (scaled charge of −0.75), many of the properties of aqueous NaCl, KCl, NaOH, and KOH solutions, such as densities, viscosities, and interfacial tensions, and their temperature dependence can be accurately computed. ,,,, Force fields with integer charges of ions (e.g., +1/–1 for Na + /Cl – ), such as the Joung–Cheatam force field, significantly overestimate the change in liquid-phase viscosities and ion diffusivities in concentrated solutions (i.e., close to the solubility limit) with respect to the pure solvents . The infinite dilution free energies of hydration of salts can be accurately captured using available integer charge force fields, whereas scaled charge force fields of ions deviate by ca.…”

“… 8 , 13 Molecular simulation is a powerful tool for atomistic modeling and predicting thermodynamic and transport properties of aqueous electrolyte solutions at different temperatures, pressures, and electrolyte concentrations. 14 − 17 The accuracy of molecular simulations depends upon the potential energy surface (PES) that is used to compute the interactions between different species. 18 − 21 The PES of aqueous electrolyte solutions can be computed from ab initio calculations or semi-empirical force fields.…”

“… 42 , 43 Using the “scaled charge” force fields of Madrid-2019 40 (scaled charges of +0.85/–0.85), Madrid-Transport (scaled charges of +0.75/–0.75), 41 and the Delft Force Field of OH – (DFF/OH – ) 14 (scaled charge of −0.75), many of the properties of aqueous NaCl, KCl, NaOH, and KOH solutions, such as densities, viscosities, and interfacial tensions, and their temperature dependence can be accurately computed. 14 , 16 , 17 , 40 , 41 Force fields with integer charges of ions (e.g., +1/–1 for Na + /Cl – ), such as the Joung–Cheatam force field, 44 significantly overestimate the change in liquid-phase viscosities and ion diffusivities in concentrated solutions (i.e., close to the solubility limit) with respect to the pure solvents. 40 The infinite dilution free energies of hydration of salts can be accurately captured using available integer charge force fields, whereas scaled charge force fields of ions deviate by ca.…”