Comment on “Experimental and molecular modeling of the structure and electrical conductivity of bulk ionic aqueous solutions: sodium, potassium, and lithium chloride.”

This Comment examines the conclusions of a recent molecular dynamics computer simulation study by Amaro-Estrada et al. of cation intra-(or self-)diffusion and solution electrical conductivity in aqueous solutions of lithium, sodium and potassium chlorides. These solutions have been well-studied by experiment for more than a century, but no comparison is made with experimental literature data. Such comparison made here shows that the simulations do not reproduce the experimental trends for the intra-diffusion coefficients in these systems and that the claim that the conductivity can be predicted from the latter using the simple Nernst-Einstein equation is false. Ion-ion velocity cross-correlations cannot be neglected. Experimental data for all the aqueous alkali metal chlorides and hydrogen chloride are compared to illustrate overall trends that might guide future computational studies. In addition, it is shown that limiting ionic conductivities in water, and hence ion tracer diffusion coefficients at infinite dilution, are best fit by the Litovitz equation between 0 and 100 °C, with an exponential dependence on T −3 , rather than T −1 , as given by the Arrhenius equation. • Molecular dynamics simulations in alkali chloride solutions contradict experiment • Experimental transport properties of aqueous alkali metal chlorides examined • Ion velocity cross-correlations must be considered in computing conductivity • Limiting ionic conductivities in water fit the Litovitz equation