Molecular Dynamics Simulations of Ion Transport in Carbon Nanotube Channels

Abstract

We present a computational modeling study examining ion transport dynamics of aqueous electrolytes under severely confined conditions. Ionic current and solvent transport through carbon nanotubes in an external electric field are studied using all atom molecular dynamics simulations. Specifically, we have examined the behavior of sodium and chloride ions in nanotubes of different radii to assess the influence of confinement on the ionic current. We find a linear relationship between the current computed and potential applied for the wider nanotubes; however, there is a significant departure from linearity when the tube diameter becomes comparable to the size of the solvated ion. For the smallest tubes studied, the energy penalty to access the pore interior is too great for most ions, leading to minimal current. We provide analyses of the energy barriers associated with ion entry as well as the hydration shell properties, which supports the absence of ionic current in the smallest carbon nanotubes. ■ INTRODUCTIONThe exceptional properties of carbon nanotubes 1 (CNTs) have garnered significant attention in recent years. The ability to tune the size and material properties of CNTs makes them attractive components for nanodevices. Broad investigations have led to a number of practical applications ranging from novel subnanometer supercapacitors, 2 hydroelectric voltage generation, power converters, 3 transformative technologies for chemical separations and desalinations of water, 4 to a variety of biomedical applications such as nanoscale pipes, 5 devices to mimic fluidic transport in biological channels, 6 water pumps, 7 carbon nanotube membranes in microfluidics to control electro-osmotic flow, 8 electrophoretic transport of RNA 9 and DNA, 10 drug delivery, 11,12 etc. In particular, there have been a number of computational studies on water transport through CNTs that provide fundamental insights into confined water behavior and explore the extent that environment can alter this behavior. Examples include research on water conduction through the hydrophobic CNTs, 13 solvent kinetics of filling and emptying of CNTs, 14 water flux through modified and unmodified CNTs, 15 and water ordering in the nanotubes. 16,17 However, in many physical systems, the fluids of interest are not pure water, but solutions whose collective behavior under confinement can be drastically different. A number of factors contribute to the discrepancies, and detailed computational studies are often required to obtain a clear understanding of the observed physical and chemical phenomena. 18−20 In this work, we have concentrated on ion transport behavior in a hydrophobic environment, a pristine CNT. We utilize a set of seven different armchair carbon nanotubes 40 Å in length with radii ranging from ∼4.1−8.2 Å to provide varying degrees of confinement. 21 The CNTs have been solvated in a water box with a 1 M concentration of NaCl. The simulated system, comprised of a carbon nanotube, ions, and the water box, is shown as Figure 1. Detailed syste...

Intra-cage dynamics of molecular hydrogen confined in cages of two different dimensions of clathrate hydrates

“…Our study reveals large quantitative and qualitative impact of the dimension and finer details of the confinement structure on functionally relevant dynamic behavior of the stored molecules. Similar effects can play a role in recently observed strong decrease of the ionic current and the non-linear dependence between the current and applied potential for smaller pores in ionic systems confined in porous carbon 37 38 . Confined ions can become localized in the potential wells as the pore diameter becomes comparable to the size of the solvated ion thus higher energy penalty is needed to extract ions from the pore.…”

Calculating Single-Channel Permeability and Conductance from Transition Paths

“…Most other MD studies 22,25,28,57 also predicted very low conductance for CNTs with diameters less than 1.0 nm. The source of such discrepancy should be further investigated in future studies.…”

“…For the (8,8) CNTs used as the test case in this study, our calculations predict a picosiemensconductance at a physiological KCl concentration, which is similar to the single-channel conductance 9 of some biological ion channels 41 . In this regard, other simulation studies 23,57 indicated that the ion ( , ) conductance depends sensitively on the size (diameter) of the Na + K + CNT. Furthermore, given that charged functional groups are an important factor for the transport properties of CNTs, 24,32 here we evaluated the effect of adding one carboxylate group to the (8,8) CNT.…”

Effect of bending on the molecular transport along carbon nanotubes

“…Thus, if the nanotube cannot be fixed, the nanotube–fullerene system without an external excitation is not suited for oscillator applications. However, besides their use in oscillators, nanotubes can also be used as the channel for transporting different types of molecules . It is clear from Fig.…”

“…These properties have qualified them as a useful candidate for building integrated chips, biosensors, ion‐transport channels, nanoelectromechanical systems (NEMSs), and for a varied range of other usages . The latest technology of creating small open‐ended pipes out of nanotubes has also made them suitable as a channel for the transport of water, ions, and some other molecules like proteins, DNA, and peptides for the purpose of drug delivery . Geng et al reported stochastic nature of the transport of water, protons, small ions, and DNA through “CNT porins” and showed that the conductance and ion selectivity of the channel can be controlled in presence of local channel and membrane charges .…”