Fast Permeation of Small Ions in Carbon Nanotubes

Abstract

Simulations and experiments have revealed enormous transport rates through carbon nanotube (CNT) channels when a pressure gradient drives fluid flow, but comparatively little attention has been given to concentration‐driven transport despite its importance in many fields. Here, membranes are fabricated with a known number of single‐walled CNTs as fluid transport pathways to precisely quantify the diffusive flow through CNTs. Contrary to early experimental studies that assumed bulk or hindered diffusion, measurements in this work indicate that the permeability of small ions through single‐walled CNT channels is more than an order of magnitude higher than through the bulk. This flow enhancement scales with the ion free energy of transfer from bulk solutions to a nanoconfined, lower‐dielectric environment. Reported results suggest that CNT membranes can unlock dialysis processes with unprecedented efficiency.

Two-dimensional monolayer salt nanostructures can spontaneously aggregate rather than dissolve in dilute aqueous solutions

“…For instance, numerous experiments and molecular dynamics (MD) simulations revealed that nanoconfined water may freeze into various one-dimensional (1D) and two-dimensional (2D) polymorphous and polyamorphous structures at low temperatures 4,8,10,[16][17][18][19][20][21][22][23][24][25][26][27][28][29] . Fast mass transport and high proton conductivity were also observed for water confined inside nanotubes 2,7,30 . Notably, previous computational and experimental studies demonstrated that the static dielectric constant of water can exhibit marked decrease under strongly confined conditions [31][32][33] , which may induce unusual behaviors of ions in the strongly confined water.…”

“…A queous solutions under nanoscale confinement have attracted considerable interest over the past few years, owing to their unusual structural, dynamical, and physicochemical properties (different from those of their bulk counterparts), as well as to their broad significance for nanoscale chemical, biological, and physical systems such as ion channels/ batteries and water desalination [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] . For instance, numerous experiments and molecular dynamics (MD) simulations revealed that nanoconfined water may freeze into various one-dimensional (1D) and two-dimensional (2D) polymorphous and polyamorphous structures at low temperatures 4,8,10,[16][17][18][19][20][21][22][23][24][25][26][27][28][29] .…”

“…The dehydration of ions within the nanopores may lead to unusual physical behaviors, not observed in the corresponding bulk solutions 34,[42][43][44][45][46][47][48][49][50] . Either suppressed or enhanced ionic mobilities were reported for aqueous ionic solutions confined into nanopores of different size 7,45,50,51 . MD simulations showed that Li + and Na + ions can diffuse faster than water molecules in nanoslits, owing to frequent lateral hopping of the ions inside the bilayer solid-like water phase 44 .…”

Influence of effective polarization on ion and water interactions within a biomimetic nanopore

“…Certain aspects of such pores can be effectively mimicked by simple non-biological structures, for example graphene nanopores and metal-organic structures (2). Carbon nanotubes (CNTs) are also particularly attractive as structural templates because they can imitate many fundamental aspects of such pores including high transport efficiency, tunable pore diameters, functionalization, and well-defined hydrophobic interiors (45)(46)(47)(48)(49)(50). The relevant properties of CNT nanopores have been extensively studied, both experimentally and with simulations (49,(51)(52)(53).…”

Rational ion transport management mediated through membrane structures

“…Owing to the introduction of nanochannel structures in the desired membranes, selective ion separation can be achieved by mediating the nanochannel shape and size, charge, wettability, and specific recognition of the nanochannel walls. Membranes such as 1D CNTs [114,280] and polyhydrazide nanotubes, [281] 2D layered GO, [61,282,283] MOFs, [60,154,284] and COFs [58] consisting of interconnected and uniform pores and ultrathin [285] and sandwich-like structures [38] have been used for controllable ion sieving. For instance, stacked graphene and GO membranes with sub-nanometer interlayer spacing showed selective transport of alkali and alkaline earth cations across the membrane owing to the synergistic effect of cation-π interactions and ion dehydration.…”