Ion Transport in Electrically Imperfect Nanopores

Ionic transport through a charged nanopore at low ion concentration is governed by the surface conductance. Several experiments have reported various power-law relations between the surface conductance and ion concentration, i.e., G(surf) proportional to c(0)(alpha). However, the physical origin of the varying exponent, alpha, is not yet clearly understood. By performing extensive coarse-grained Molecular Dynamics simulations for various pore diameters, lengths, and surface charge densities, we observe varying power-law exponents even with a constant surface charge and show that alpha depends on how electrically perfect the nanopore is. Specifically, when the net charge of the solution in the pore is insufficient to ensure electro-neutrality, the pore is electrically imperfect and such nanopores can exhibit varying alpha depending on the degree of imperfectness. We present an ionic conductance theory for electrically imperfect nanopores that not only explains the various power-law relationships but also describes most of the experimental data available in the literature.