Length scales in electrolytes

Abstract

The elusive presence of an anomalously increasing screening length at high ionic concentrations hampers a complete image of interactions in electrolytes. Theories that extend the diluted Debye–Hückel framework to higher concentrations predict, in addition to the expected decreasing Debye length, an increasing significant scale of the order of at most a few ionic diameters. More recent surface force balance experiments with different materials succeeded in measuring increasing length scales that, however, turned out to extend over tenths or even hundreds of ionic diameters. While simulation work has managed to characterize the former, the latter still avoids detection, generating doubts about its true origin. Here, we provide a step forward in the clarification of such a conundrum. We have studied by extensive molecular dynamics simulation the properties of a generic model of electrolyte, lithium tetrafluoroborate dissolved in ethylene-carbonate, in a vast range of salt concentrations continuously joining the Debye non-interacting limit to the opposite overcharged solvent-in-salt states. On one side, we have accurately determined the macroscopic concentration-induced structural, dielectric, and transport modifications; on the other, we have quantified the resulting nanoscale ion organization. Based only on the simulation data, without resorting to any uncontrolled hypotheses or phenomenological parameters, we identify a convincing candidate for the measured anomalously increasing length, whose origin has possibly been misinterpreted.