Abstract
The electrostatic screening length predicted by Debye-Hückel theory decreases with increasing ionic strength, but recent experiments have found that the screening length can instead increase in concentrated electrolytes. This phenomenon, referred to as underscreening, is believed to result from ion-ion correlations and short-range forces such as excluded volume interactions among ions. We use Brownian Dynamics to simulate a version of the Restrictive Primitive Model for electrolytes over a wide range of ion concentrations, ionic strengths, and ion excluded volume radii for binary electrolytes. We measure the decay of the charge-charge correlation among ions in the bulk and compare it against scaling trends found experimentally and determined in certain weak coupling theories of ion-ion correlation. Moreover, we find that additional large scale ion structures emerge at high concentrations. In this regime, the frequency of oscillations computed from the charge-charge correlation function is not dominated by electrostatic interactions but rather by excluded volume interactions and with oscillation periods on the order of the ion diameter. We also find the nearest neighbor correlation of ions sharing the same charge transitions from negative at small concentrations to positive at high concentrations, representing the formation of small, like-charge ion clusters. We conclude that the increase in local charge density due to the formation of these clusters and the topological constraints of macroscopic charged surfaces can help explain the degree of underscreening observed experimentally.
Highlights
The study of concentrated electrolytes has recently drawn considerable interest due to their central importance in various applications, ranging from colloidal self-assembly1,2 and biological processes3 to supercapacitors and batteries.4 The delicate balance of long-range electrostatic interactions and steric repulsion poses a physically complex problem
We use Brownian Dynamics coupled with a truncated multipole expansion of the electric potential to study the structure of concentrated electrolytes using a version of the Restrictive Primitive Model
We focus on the decay of spatial correlation functions and extract the inverse of the correlation length, κ, and frequency of oscillation, ω
Summary
The study of concentrated electrolytes has recently drawn considerable interest due to their central importance in various applications, ranging from colloidal self-assembly and biological processes to supercapacitors and batteries. The delicate balance of long-range electrostatic interactions and steric repulsion poses a physically complex problem. Stout and Khair used Bikerman and Carnahan–Starling type models to account for the entropic effects due to ion size to study the importance of steric interactions on diffusiophoresis in concentrated electrolytes These local density approximations are unable to capture the charge density oscillations present in concentrated electrolytes, do not show underscreening, and have been found to be ill-equiped to accurately describe the electric double layer structure.. Adar et al. modified the classical Coulomb potential to account for the finite volume over which an ion’s charge is distributed and examined its importance in setting the correlation length for charge–charge interactions We refer to this model as the “shell model” throughout our analysis. We perform a cluster analysis on likecharge ions in our model and compare the cluster size probability distribution against that of a binary hard sphere liquid, thereby revealing new insight into the structure of concentrated electrolytes in bulk
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