Rectification of bipolar nanopores in multivalent electrolytes: effect of charge inversion and strong ionic correlations

TL;DR

This paper investigates how strong ionic correlations in multivalent electrolytes affect the rectification behavior of bipolar nanopores, revealing phenomena like charge inversion and leakage not captured by mean-field models.

Contribution

It demonstrates that ionic correlations lead to nonmonotonic rectification effects and charge inversion, which are not predicted by traditional mean-field theories, using both PNP and LEMC methods.

Findings

Ionic correlations cause coion leakage absent in mean-field models.

Charge inversion and overcharging are observed with LEMC but not with PNP.

Rectification behavior depends nonmonotonically on surface charge strength.

Abstract

Bipolar nanopores have powerful rectification properties due to the asymmetry in the charge pattern on the wall of the nanopore. In particular, bipolar nanopores have positive and negative surface charges along the pore axis. Rectification is strong if the radius of the nanopore is small compared to the screening length of the electrolyte so that both cations and anions have depletion zones in the respective regions. The depths of these depletion zones is sensitive to sign of the external voltage. In this work, we are interested in the effect of the presence of strong ionic correlations (both between ions and between ions and surface charge) due to the presence of multivalent ions and large surface charges. We show that strong ionic correlations cause leakage of the coions, a phenomenon that is absent in mean field theories. In this modeling study, we use both the mean-field Poisson-Nernst-Planck (PNP) theory and a particle simulation method, Local Equilibrium Monte Carlo (LEMC), to show that phenomena such as overcharging and charge inversion cannot be reproduced with PNP, while LEMC is able to produce nonmonotonic dependence of currents and rectification as a function of surface charge strength.