Ion Pairing in Aqueous Lithium Salt Solutions with Monovalent and Divalent Counter-Anions

Molecular dynamics simulations of concentrated aqueous solutions of LiCl and Li2SO4 were conducted in order to provide molecular insight into recent neutron scattering data. The structures predicted from the molecular dynamics simulations using standard nonpolarizable force fields provided a very poor fit to the experiment; therefore, refinement was needed. The electronic polarizability of the medium was effectively accounted for by implementing the electronic continuum correction, which practically means rescaling the ionic charges. Consistent with previous studies, we found that this approach in each case provided a significantly improved fit to the experimental data, which was further enhanced by slightly adjusting the radius of the lithium ion. The polarization effect was particularly pronounced in the Li2SO4 solution where the ions in the nonpolarizable simulations tended to cluster unphysically. With the above alterations, the employed force field displayed an excellent fit to the neutron scattering data and provided a useful interpretative framework for the experimental measurements. At the same time, the present study underlines the importance of solvent polarization effects in hydration of ions with high charge density.