Structural and Dynamical Crossovers in Dense Electrolytes

TL;DR

This study uses molecular dynamics simulations to explore how increasing salt concentration causes structural and dynamical crossovers in dense electrolytes, revealing distinct regimes and the importance of ion-solvent interactions.

Contribution

It provides a detailed analysis of structural and dynamical crossovers in dense electrolytes, highlighting the role of ion-solvent coupling and identifying diffusion-corrected ion-pair lifetime as a key descriptor.

Findings

Screening transition from dilute to concentrated regimes

Discontinuous dynamical crossovers in ion diffusion and pairing

Percolation transition at higher ionic concentrations

Abstract

Electrostatic interactions fundamentally govern the structure and transport of electrolytes. In concentrated electrolytes, however, electrostatic and steric correlations, together with ion-solvent coupling, give rise to complex behavior, such as underscreening, that remains challenging to explain despite extensive theoretical effort. Using molecular dynamics simulations of primitive electrolytes with and without space-filling solvent particles, we elucidate the structural and dynamical crossovers and their connection that emerge with increasing salt concentration. Explicit-solvent electrolytes exhibit a screening transition from a charge-dominated dilute regime to a density-dominated concentrated regime, accompanied by dynamical crossovers in ion self-diffusion and ion-pair lifetimes. These dynamical crossovers display a marked discontinuity, unlike the smoother variation of the screening crossover, which originates from short-range ion-counterion structures. The pronounced growth of ionic clusters leads to a percolation transition only at higher concentrations, whereas the onset of the structural and dynamical crossovers is associated primarily with local ion pairing or small aggregates. Both structural and dynamical behaviors are found to depend sensitively on ion-solvent coupling: implicit-solvent electrolytes exhibit a screening transition between two charge-dominated regimes, accompanied by qualitatively distinct dynamical behavior. Finally, we demonstrate that the diffusion-corrected ion-pair lifetime provides a consistent descriptor linking ionic structure and dynamics across electrolyte systems.