Carbon nanotubes are promising materials with a vast range of potential applications, such as nanoscale electronic devices, nanoscale sensors, selective molecular filters, gas storage devices, nanofluidic devices, and targeted drug delivery devices, among many others. Water has unique properties and an understanding of the interaction between water and carbon nanotubes is key to the development of many of these devices. The confinement of water in carbon nanotubes as compared to bulk water has been shown to produce some remarkable features, which include but are not limited to a lower number of hydrogen bonds, the increased lifetime of hydrogen bonds, the layered structure of water under confinement, its reduced density and viscosity, and an increased proton mobility in 1-D water chains. One of the more exciting results of water under confinement is the high slip lengths that result at high flow rates, which is conducive to ultrafiltration and nanofiltration. High water flow rates along with selective ion transport is an important feature of ion channels in living organisms; however experimental analysis to study ion channels is a real challenge as ion channels can degrade under experimental conditions, have dimensions of the order of few nanometers, possess non-uniform surfaces, and uneven charge distributions that make them a complex system to study. Carbon nanotubes can be easily embedded into lipid membranes, and hence can be used as biomimetic devices to study water and ion transport at nanoscopic levels.
Flow of water and ions in nanotubes can be achieved by application of an electric field, pressure difference, concentration gradient, uneven charge distribution, and osmotic pressure. The rate and selectivity of ion transport can be controlled by various means such as varying the pore size, the ion concentration, the charge distribution, the density of charge and through chemical functionalization. Because of the large surface to volume ratio as compared with macro and microscopic length scales, surface charge has a pronounced effect on the fluid volume in the hydrophobic cavity at the nanoscopic length scale, leading to a rejection of the co-ions. This phenomenon can thus help improve the selectivity of the ions beyond steric hindrance to include the use of larger diameter nanotubes enabling greater water eflux, which can be exploited to model nano-level filters. Since continuum Navier-Stokes equations for fluid transport break down at nanometer level, molecular dynamics (MD) simulations, which explicitly calculate the motion of all particles in the system described by Newton's equations of motion, is the favored method of study.
Water and ion transport through uncharged and charged carbon nanotube membranes (d = 0.8nm -3.25nm) were studied using long-time MD simulations. The OPLS force field parameters of benzene are employed for carbon atoms. The carbon atoms were held fixed during the course of the simulation. The TIP4P water potential was used to model the water intermolecular interactions. All simulations were carried out in GROMACS 4.6.1. in the NVT ensemble. Various cases were simulated by varying pore size at different electrolyte (NaCl) concentrations and application of electric field for both charged and uncharged CNT membranes at these different pore sizes and concentrations. For uncharged CNTs in the absence of electric field, steric hindrance is the only factor controlling transport of ions. For small pore diameters (<0.8nm) the ion occupancy and passage of ions through the tube is negligible as the hydration radius of the ions is comparable to the pore size. In this case, water diffuses to the chamber containing electrolyte through osmosis. At larger pore sizes the concentration gradient drives ion transport with ion occupancy in the CNT and ion passage progressively increasing. In the case of uncharged CNTs in the presence of electric field, electro-osmosis dominates with increasing field strength. In charged CNTs at low electrolyte concentrations, the thickness of the electric double layer is of the same order as the diameter of the pore and this results in selectivity governed by the Donnan equilibrium. As the concentration strength increases the Debye length becomes smaller, the charges are screened and both ions can pass through. Electrokinetic flow is observed in the case of charged CNT under an electric field, which can be described by the Poisson-Boltzmann equation along with the Nernst-Planck equation. Modulating the strength of the surface charges can be used to tune both ion transport and water flow through the nanotubes.
The structural characteristics of water and ions confined in CNT for the various cases simulated are compared. Measurements of ion occupancy in the nanotube, water flux, ion flux, rejection rates, and other transport properties are obtained using MD simulations. A complete account of ion and water transport under the influence of charge is obtained in this study spanning a large number of CNT diameters. Fast water flow can thus be integrated with ion selectivity through modulation of surface charges and pore size, which not only helps to understand biological processes, but can also help design advanced nanofluidic devices in the future.
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