Enhanced transport of Li⁺ over other alkali ions in small-diameter carbon nanotubes

Ion transport in angstrom-scale 1-D pores is of interest to understanding biological channels, as well as for applications in separation membranes. In such confined pores, hydrated ions undergo partial or complete dehydration, where water molecules around the ions are reoriented or stripped away in order to enter the pores. Here, we describe experiments comparing the pressure- and electric-field- driven transport of various alkali ions in 0.8-nm and 3-nm carbon-nanotube CNT pores. In both types of nanotubes, the transport is selective to cations due to positively charged functional groups at the nanotube tips. However, in 0.8-nm pores, both the pressure-driven streaming current and the ionic conductance are greater for Li⁺ ions than for K⁺ ions, which is a reversal of their relative mobilities in bulk solution. The enhanced transport of Li⁺ over other alkali ions is consistent in 0.8-nm CNT pores regardless of the concentration of the alkali metal chloride solutions. In contrast, for 3-nm CNTs pores, the faster relative transport of Li⁺ compared to K⁺ only occurs below a certain concentration, and reverses at higher concentrations. We attribute this behaviour to the varying thickness of the electric double layer as compared to the pore diameter. Better fundamental understanding of the complex interactions between ions and 1-D nanopores may enable rational design of membranes for lithium recovery, desalination, and other separation applications.