Electroneutrality Breakdown and Perm-Selective End Groups in Carbon Nanotube Porins

Abstract

Sub-nanometer carbon nanotube porins (n-CNTP) have been shown to achieve high water permeation with significant permselectivity precluding anion transport. Therefore, n-CNTPs are useful model systems to probe and optimize the nanoscale transport phenomena relevant for selective ionic separations. Surprisingly, these n-CNTPs demonstrate a unique conductance scaling behavior with ionic concentration, which has led to various continuum models being constructed in pursuit of determining the underpinning physics governing nanoscale ionic transport. Here, we formulate a continuum model to capture the conductance and permselectivity of n-CTNPs considering the breakdown of electroneutrality within the pore. Electroneutrality breakdown occurs when the ionic charge within the pore is not balanced by the charge on the pore walls. While electroneutrality is commonly assumed in the analysis of nanopore transport, we predict a departure from electroneutrality due to the extreme one-dimensional confinement in n-CNTPs, strongly influencing the conductance and selectivity of the pore. Furthermore, our analysis demonstrates the importance of the entrance groups in modulating ion transport through n-CNTPs. Lastly, we provide a framework for future refinements to incorporate additional microscopic physics into the phenomenological model.