Selective transport of alkali ions in vertically aligned sub-nanometer-diameter carbon-nanotube membranes*

Normally, the ion conductance of different ions through a porous membrane is proportional to the ion mobility. For alkali ions in aqueous solution, the ion mobility increases as the hydrated ionic diameter decreases, e.g., with µ_K+ > µ_Na+ > µ_Li+. This phenomenon remains valid even for porous membranes with nanoscale pores, which typically are still larger than the size of hydrated ions. However, when the pore size shrinks to the angstrom scale, alkali ions can undergo partial or complete dehydration, in which water molecules around the ions are re-oriented or stripped away in order to enter the pores. Hence, the relative ordering of the ion-conductance values for alkali ions in angstrom-scale pores can become dependent on the size of the dehydrated ions, which is entirely opposite to that of hydrated alkali ions. This mechanism has been widely confirmed in biological ion channels, but not experimentally seen in carbon nanotubes (CNTs), to our knowledge. Here, we describe ion conductance in the first macroscopic membranes having sub-nanometer-diameter, vertically aligned carbon-nanotube (CNT) pores. In such membranes with 0.8-nm CNT pores, the ion conductances of aqueous alkali metal chloride solutions followed an order reversed from the bulk ion mobilities, indicating that the ions had their hydration shells significantly re-oriented or stripped.  As a comparison, we demonstrated that the relative ion conductances of 3-nm CNT membranes remained in the order of bulk ionic mobility. These results not only verify the integrity of the scalably fabricated sub-nanometer-diameter CNT membranes, but provide an avenue to study fundamental ion-transport mechanisms in angstrom-scale 1-D pores, and for applications such as ion separation, rectification and gating.