Charge Transport in Water–NaCl Electrolytes with Molecular Dynamics Simulations

Abstract

A systematic description of microscopic mechanisms is necessary to understand mass transport in solid and liquid electrolytes. From Molecular Dynamics (MD) simulations, transport properties can be computed and provide a detailed view of the molecular and ionic motions. In this work, ionic conductivity and transport numbers in electrolyte systems are computed from equilibrium and nonequilibrium MD simulations. Results from the two methods are compared with experimental results, and we discuss the significance of the frame of reference when determining and comparing transport numbers. Two ways of computing ionic conductivity from equilibrium simulations are presented: the Nernst−Einstein approximation or the Onsager coefficients. The Onsager coefficients take ionic correlations into account and are found to be more suitable for concentrated electrolytes. Main features and differences between equilibrium and nonequilibrium simulations are discussed, and some potential anomalies and critical pitfalls of using nonequilibrium molecular dynamics to determine transport properties are highlighted.

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

“…Surprisingly we do not obtain better results than those of the unit charge models. This was also observed by Gullbrekken et al . in their recent work in which they studied the electrical conductivities by using an integer charge force field but then when scaling the charges they did not observe differences.…”

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

“…The initial increase in electrical conductivities is due to the increase in the number of charge carriers, while the subsequent decline is attributed to the increase in the viscosity (and decrease in ion diffusivities) at higher salt molalities. 76 This behavior has also been observed for other electrolyte solutions. 99,100 The decrease in the self-diffusivities of Na + and B(OH) 4…”

“…In the NVT ensemble, an equilibration run of 1 ns and a production run of 20–50 ns is used. All self-diffusivities shown here are corrected for finite-size effects using the Yeh–Hummer equation. − Dynamic viscosities computed using MD are not subject to finite-size effects. ,,, The electrical conductivities (κ) in this work are computed using the Einstein–Helfand approach. , κ = e 2 N V k normalB T ∑ i , j z i z j normalΛ i j where e is the elementary charge, N is the total number of molecules, V is the volume of the simulation box, k B is the Boltzmann factor, and T is the absolute temperature. z i and z j are the charges of ions of type i and j , respectively.…”

“…69,72,74,75 The electrical conductivities (κ) in this work are computed using the Einstein−Helfand approach. 76,77 = e N Vk T z z…”

“…NaCl. 76,81 At the limit of low NaB(OH) 4 molalities (m → 0), the NE expression and the exact expression (eq 1) are identical by definition. This is (by definition) not the case for exact expression (eq 1) and the NE expression with the Yeh− Hummer correction 70,71 ionic diffusivities.…”