Screening Lengths in Ionic Fluids

TL;DR

This paper demonstrates that ionic fluids with a solvent can exhibit multiple decay lengths in correlation functions, challenging the traditional view of a single decay length, and explains recent experimental observations.

Contribution

The study introduces an analytical theory and molecular dynamics simulations showing multiple decay lengths in ionic fluids with solvents, revising classical screening length assumptions.

Findings

Multiple decay lengths coexist in ionic fluids with solvents.

Discontinuous changes in structural force are explained by varying composition.

Reframes debates on screening length in concentrated electrolytes.

Abstract

The decay of correlations in ionic fluids is a classical problem in soft matter physics that underpins applications ranging from controlling colloidal self-assembly to batteries and supercapacitors. The conventional wisdom, based on analyzing a solvent-free electrolyte model, suggests that all correlation functions between species decay with a common decay length in the asymptotic far field limit. Nonetheless, a solvent is present in many electrolyte systems. We show using an analytical theory and molecular dynamics simulations that multiple decay lengths can coexist in the asymptotic limit as well as at intermediate distances once a hard sphere solvent is considered. Our analysis provides an explanation for the recently observed discontinuous change in the structural force across a thin film of ionic liquid-solvent mixtures as the composition is varied, as well as reframes recent debates in the literature about the screening length in concentrated electrolytes.