(204c) 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.