The effects of electrostatic correlations on the ionic current rectification in conical nanopores

Abstract

Ion‐ion electrostatic correlations are recognized to play a significant role in the presence of concentrated multivalent electrolytes. To account for their impact on ionic current rectification phenomenon in conical nanopores, we use the modified continuum Poisson‐Nernst‐Planck (PNP) equations by Bazant et al. Coupled with the Stokes equations, the effects of the EOF are also included. We thoroughly investigate the dependence of the ionic current rectification ratios as a function of the double layer thickness and the electrostatic correlation length. By considering the electrostatic correlations, the modified PNP model successfully captures the ionic current rectification reversal in nanopores filled with lanthanum chloride LaCl3. This finding qualitatively agrees with the experimental observations that cannot be explained by the standard PNP model, suggesting that ion‐ion electrostatic correlations are responsible for this reversal behavior. The modified PNP model not only can be used to explain the experiments, but also go beyond to provide a design tool for nanopore applications involving multivalent electrolytes.

Scaling Behavior of Bipolar Nanopore Rectification with Multivalent Ions

“…The importance of multivalent ions in achieving peculiar conductance behavior of nanopores due, for example, to charge inversion or charge selectivity, is well-known. − Still, the number of papers dealing with multivalent electrolytes (either z + > 1 or | z – | > 1) compared to NaCl or KCl is relatively small, also see refs − .…”

Diffusioosmotic flow reversals due to ion–ion electrostatic correlations

“…This sign change in has been reported in prior work and causes various anomalous electrokinetic phenomena, including electroosmotic flow reversal, 70 electrophoretic mobility reversal, 72 dielectrophoretic polarization reversal, 73 and ionic current rectification reversal. 74 In sum, electrostatic correlations cause overscreening of the channel surface charge that leads to a sign change in the space charge density near the channel surface. Such a sign change in the space charge density in turn causes a reversal in the electroosmotic driving force that generates the electroosmotic flow, leading to a sign reversal in the diffusioosmotic mobility.…”

“…For instance, the modified Poisson equation can successfully capture non-equilibrium phenomena that follow from overscreening, including electroosmotic flow reversal, 70 electrophoretic mobility reversal, 72 dielectrophoretic polarization reversal, 73 and ionic current rectification reversal. 74 In this article, we develop a mathematical model that predicts the diffusioosmotic mobilities of binary symmetric electrolytes across low to high concentrations in a channel comprising two charged parallel plates. We use the modified Poisson equation 69 to model the electric potential and electrostatic correlations.…”

“…For instance, the modified Poisson equation can successfully capture non-equilibrium phenomena that follow from overscreening, including electroosmotic flow reversal, 70 electrophoretic mobility reversal, 72 dielectrophoretic polarization reversal, 73 and ionic current rectification reversal. 74…”