Putting Together the Puzzle of Ion Transfer in Single-Digit Carbon Nanotubes: Ab Initio Meets Mean-Field

Abstract

Nature employs channel proteins to selectively pass water across cell membranes, which inspires search for bio-mimetic analogues. Carbon nanotube porins (CNTPs) are intriguing mimics of water channels, yet ion transport in CNTPs still poses questions. As alternative to continuum models, here we present a molecular mean-field model, computing ab initio all required thermodynamic quantities for KCl salt and H+ and OH- ions present in water. Starting from water transfer, the model considers transfer of free ions, along with ion-pair formation to address ion-ion interactions. High affinity to hydroxide, suggested by experiments and making it dominant charge carrier in CNTP, is revealed as an exceptionally favorable transfer of KOH pairs. Nevertheless, free ions, coexisting with less mobile ion-pairs, apparently control ion transport. The model explains well the observed effects of salt concentration and pH on conductivity, transport numbers, anion permeation and its activation energies, and current rectification. The proposed approach is extendable to other sub-nanochannels and help design novel osmotic materials and devices.