Proton Transport through Water-Filled Carbon Nanotubes

Abstract

Proton transfer along 1D chains of water molecules inside carbon nanotubes is studied by simulations. Ab initio molecular dynamics and an empirical valence bond model yield similar structures and time scales. The proton mobility along 1D water chains exceeds that in bulk water by a factor of 40, but is reduced if orientational defects are present. Excess protons interact with hydrogen-bonding defects through long-range electrostatics, resulting in coupled motion of protons and defects.

Self-assembling subnanometer pores with unusual mass-transport properties

“…For instance, compared with K + conductance that requires physical diffusion, proton transport could be much more efficient based on an orientational hydrogen-bonding defect mechanism 50,51 . Here the observed K + /Na + selectivity can be understood based on conclusions from previous MD simulations 48,49 , which demonstrates that the free-energy costs of constraining hydrated K + within a sub-nm pore are notably less than those of hydrated Na + .…”

Diameter-dependent ion transport through the interior of isolated single-walled carbon nanotubes

“…In pore-blocking events, fast proton flux 23 through the interior of SWNT results in a high current level, depleting the proton concentration in the near-pore region and increasing the concentration of cations near the pore mouth (Fig. 2a).…”

“…Water confined in nanopores of diameter similar to its molecular size demonstrates slip-flow at the walls 31 , reduced viscosity 42 and increased proton transport rates 23 . Both protons and blocking ions within a SWNT will feel these effects and our simple model, illustrated in Fig.…”

“…S12). We approximate the proton mobility with an exponential interpolation between the small diameter-enhanced rate mobility 23 and bulk transport rates 43 . We note that most of the properties are strongly diameter dependent, currently not well-understood, and with varying values in literature, as only simulation results have been available to date without an experimental platform for validation.…”

“…These properties potentially allow for interesting desalination 22 and DNA-sequencing applications 13 , as well as the ability to probe basic fluid structure properties at the smallest of possible scales. For this reason, a large number of molecular simulations have demonstrated interesting diameterdependent phenomena such as ion selectivity 15,22 , rapid proton conduction 18,23 that can be caused by the unique water structures and geometric confinements [24][25][26] , and altered water-ice phase behaviour 27 . However, most studies have focused on theoretical simulations, and their experimental verification is still a subject of debate [28][29][30][31] .…”