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

In this work, we computed electrical conductivities underambientconditions of aqueous NaCl and KCl solutions by using the Einstein-Helfandequation. Common force fields (charge q = & PLUSMN;1 e) do not reproduce the experimental values of electricalconductivities, viscosities, and diffusion coefficients. Recently,we proposed the idea of using different charges to describe the potentialenergy surface (PES) and the dipole moment surface (DMS). In thiswork, we implement this concept. The equilibrium trajectories requiredto evaluate electrical conductivities (within linear response theory)were obtained by using scaled charges (with the value q = & PLUSMN;0.75 e) to describe the PES. The potentialparameters were those of the Madrid-Transport force field, which accuratelydescribe viscosities and diffusion coefficients of these ionic solutions.However, integer charges were used to compute the conductivities (thusdescribing the DMS). The basic idea is that although the scaled chargedescribes the ion-water interaction better, the integer chargereflects the value of the charge that is transported due to the electricfield. The agreement obtained with experiments is excellent, as forthe first time electrical conductivities (and the other transportproperties) of NaCl and KCl electrolyte solutions are described withhigh accuracy for the whole concentration range up to their solubilitylimit. Finally, we propose an easy way to obtain a rough estimateof the actual electrical conductivity of the potential model underconsideration using the approximate Nernst-Einstein equation,which neglects correlations between different ions.

Automated summary

In this study, the electrical conductivities of aqueous NaCl and KCl solutions under ambient conditions were computed using the Einstein-Helfand equation. Conventional force fields could not accurately predict the experimental values of electrical conductivities, viscosities, and diffusion coefficients. However, by introducing the concept of using different charges to describe the potential energy surface and the dipole moment surface, excellent agreement with experimental results was achieved for the first time for the entire concentration range.