(485a) Ion Transport through Carbon Nanotubes: A Molecular Dynamics Study

We have performed molecular dynamics simulations on single-walled carbon nanotubes (CNTs) in aqueous environments with explicit solvent to obtain an insight into the process of ion permeation through narrow hydrophobic pores. We have studied in detail the structural, dynamical and energetic aspects of the permeation mechanism as a function of CNT pore size, external solution concentrations, the number and nature of charges on the CNT wall.

While a number of simulation studies have been conducted on water transport through nanotubes, the studies on ion conduction have been limited owing to the stochastic nature and extremely long timescales of simulations required for observing ion conduction in CNTs with pore sizes of less than 1nm. Using long molecular dynamics simulations combined with free energy calculations, we attempt to clarify the phenomena associated with ion transport in charged and uncharged CNTs at the continuum limit in 2-3 nm pore diameter CNTs where the bulk-like features of water are nearly restored and ultimately provide a robust model that can help quantitatively predict transport. The model accounts for the confinement-induced structural, dynamical and energetic differences from bulk conditions that lead to a deviation of ion flux from continuum predictions of the Poisson-Nernst-Planck equation.

Since the process of ion conduction is a function of the free energy barrier, we also calculate the conduction rate from the free energy profile using umbrella sampling and the diffusion coefficients of the ionic species across the CNT membrane. We provide a comparison of the ion conductance through CNTs with experimental and simulation studies of similar biological and artificial membranes. To further gauge the quality of the energetic barriers from free energy profiles estimated from our potential of mean force studies, we also calculate the solvation energies of ions in bulk and under confinement in the CNT from a modified free energy perturbation technique, namely, the Bennett acceptance ratio method.

Lastly, we compare our results of coion exclusion from simulations to the theoretical predictions obtained from Donnan theory of ion exclusion for permselective membranes that ignore the effect of geometry and pore size, assume homogeneous distribution of surface charges and an ideal behavior of free ions in solution throughout space. We discuss the feasibility of using the thermodynamically-derived Donnan theory to make adequate predictions of coion concentrations in charged CNT-based nanoporous membranes with diameters less than 3nm.