The mixture effect on ionic selectivity and permeability of nanotubes

Ion-selective nanotubes have great potential in applications such as ion separation, desalination, and power generation. However, their performance is often limited by the deteriorated selectivity in mixed salt solutions. To reveal the underlying mechanism of the mixture effect on ion transport through nanotubes, we perform molecular dynamics (MD) simulations on ion transport through carbon nanotubes (CNTs) and polymer nanopores with a pore diameter of ∼1 nm and a charge density of -1 e nm^-2. Based on the simulation results, when a single salt solution is replaced by a mixed salt solution, the ionic selectivity drops as the permeability of higher permeable ions decreases much greater than that of lower permeable ions. This is because the adsorption of lower permeable ions on the inner surface of nanotubes blocks the ion flux and increases the entrance barrier to the nanotube, and the adsorption is significantly reduced in the mixed salt solution. Such a reduction results from the occupancy of higher permeable ions on the adsorption sites as they have a higher adsorption tendency albeit weaker adsorption compared with lower permeable ions. These studies will help design the next generation of nanostructures to circumvent the mixture effect and show high permeability and selectivity in real applications.