  The electrostatic screening length predicted by Debye-H\"uckel 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 that 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.
