Correlation Length in Concentrated Electrolytes: Insights from All-Atom Molecular Dynamics Simulations

We study the correlations length of the charge-charge pair correlations in concentrated electrolyte solutions by means of all-atom, explicit-solvent molecular dynamics simulations. We investigate LiCl and NaI in water, which constitute highly soluble, prototypical salts for experiments, as well as two more complex, molecular electrolyte systems of lithium bis(trifluoromethane)sulfonimide (LiTFSI), commonly employed in electrochemical storage systems, in water and in an organic solvent mixture of dimethoxyethane (DME) and dioxolane (DOL). Our simulations support the recent experimental observations as well as theoretical predictions of a non-monotonic behavior of the correlation length with increasing salt concentration. We observe a Debye-Hückel like regime at low concentration, followed by a minimum reached when d/λ_D = 1, where λ_D is the Debye correlation length and d the effective ionic diameter, and an increasing correlation length with salt concentration in very concentrated electrolytes. As in the experiments, we find that the screening length in the concentrated regime follows a universal scaling law as a function d/λ_D for all studied salts. However, the scaling exponent is significantly lower than the experimentally measured one, and lies in the range of the theoretical predictions based on much simpler electrolyte models.