Screening Lengths in Ionic Fluids

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.

Screening length for finite-size ions in concentrated electrolytes

“…Another possibility is to model the solvent explicitly as charge dipoles in order to account for the dielectric decrement in the presence of ions. An explicit treatment of solvent is also important for the structural force between surfaces at small separations, originating from non-electrostatic forces of concentrated solvent [41]. The compact analytical form of the correlation function [Eq.…”

“…First, it is possible to restrict the Coulomb interaction to ionic separations of r > a with a cutoff at r = a, given by v co (r) = Θ (r − a) l B /r, (for example, see Ref. [41]) where Θ(x) is the Heaviside function. It is useful to rewrite this function in Fourier space.…”

The intimate relationship between the dielectric response and the decay of intermolecular correlations and surface forces in electrolytes

“…Simultaneously, there was a monotonic long-range decay mode. Their findings regarding the oscillatory contributions (but not the long-range monotonic one) have been illustrated theoretically in calculations by Coupette et al 27,28 for a semi-primitive model of electrolyte solutions, where the monovalent ions and the solvent are hard spheres. They studied systems with equally-sized ions, 27 where the model is a charge-inversion invariant system, and systems with unequally-sized ions.…”

“…Eqn (27) is easily derived as follows. By inserting (6) into eqn (26) and then using eqn (7) we obtain…”

Simulation of a Solvate Ionic Liquid at a Polarizable Electrode with a Constant Potential

“…It will be interesting to see how this curve and capacitance, in general, would evolve with time particularly when compared to conventional ionic liquid solutions.In a somewhat related point due to the box size and simulations length, it has not been possible, as usual with simulations of this type to extract screening lengths. Recent work has however shown that concentrated electrolytes exhibit non negligible long range decay lengths ,51,52 with theoretical and computational studies supporting these conclusions 41,53,54.…”