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The electrostatic screening length predicted by Debye-H\"uckel theorydecreases with increasing ionic strength, but recent experiments have foundthat the screening length can instead increase in concentrated electrolytes.This phenomenon, referred to as underscreening, is believed to result fromion-ion correlations and short-range forces such as excluded volumeinteractions among ions. We use Brownian Dynamics to simulate a version of theRestrictive Primitive Model for electrolytes over a wide range of ionconcentrations, ionic strengths, and ion excluded volume radii for binaryelectrolytes. We measure the decay of the charge-charge correlation among ionsin the bulk, and compare it against scaling trends found experimentally anddetermined in certain weak coupling theories of ion-ion correlation. Moreover,we find that additional large scale ion structures emerge at highconcentrations. In this regime, the frequency of oscillations computed from thecharge-charge correlation function is not dominated by electrostaticinteractions but rather by excluded volume interactions and with oscillationperiods on the order of the ion diameter. We also find that the nearestneighbor correlation of ions sharing the same charge transitions from negativeat small concentrations to positive at high concentrations, representing theformation of small, like-charge ion clusters. We conclude that the increase inlocal charge density due to the formation of these clusters and the topologicalconstraints of macroscopic charged surfaces can help explain the degree ofunderscreening observed experimentally.

Underscreening and hidden ion structures in large scale simulations of concentrated electrolytes