A Molecular Dynamics Study on the Influence of Charge on the Transport of Water and Ions through Carbon Nanotubes (Award submission)

Carbon nanotubes (CNTs) are known to possess smooth hydrophobic walls, can be easily chemically functionalized, and have high surface to volume ratio, thus offering a possibility of unconventionally high flow rates and ion selectivity that makes them potential candidates for  nanofluidic and biomimetic applications. However, it is difficult to measure transport processes at nanometer scales and thus much of the experimental data on water and ion transport through nanotubes available so far is limited to larger diameter nanotubes where a predominantly bulk-like behavior is observed. Motivated by the need to understand the superior transport properties of these extremely narrow pores, we perform long time (100 ns) classical MD simulations to investigate in atomic detail the intriguing structural and dynamical features of a NaCl solution under confinement in sub-3 nm diameter carbon nanotubes. Additionally, we assess if Donnan equilibrium theory can be used to ascribe a relation between membrane fixed charges and the concentration of ions within the nanotube.

In this study, the spatial distribution of water and ions and their transport properties in terms of their apparent diffusion coefficients are investigated through uncharged and charged armchair carbon nanotubes (d = 0.8 nm – 3.0 nm) embedded between two reservoirs of a 1M NaCl solution. The influence of charges applied to the surface of the CNT on ion-exclusion/screening phenomena is quite apparent and its impact is evident when the pore sizes are comparable to the Debye lengthscale. This can be attributed to the fact that at nanoscopic length scales, because of the large surface to volume ratio as compared with macro and microscopic length scales, surface charge has a pronounced effect on fluid volume in the hydrophobic cavity leading to rejection of ions. We obtain and compare the apparent diffusion coefficients of water and ions in bulk phase to that under confinement and in the vicinity of charges.  We find that while the pore sizes that we study are permeable to ions, the axial diffusion coefficient of water and ions decreases under confinement and its effect is most prominent in the smallest diameter charged tube. We also obtain measurements of ion occupancy in the nanotube, coordination numbers of ions, water and ion flux, and ion rejection rates for the various cases considered.

The cases studied give a broad account of ion and water transport properties through uncharged and charged CNTs. The goal is to integrate fast water flow with ion selectivity through modulation of pore sizes, surface charges, and electric fields with potential applications in desalination, as molecular gates for separation of biomolecules, and other nanofluidic devices.