Ion Permeability and Selectivity in Composite Nanochannels: Engineering through the End Effects

Ion transport through nanochannels allows ultrafast permeation and highly efficient separation, becoming promising for applications in water purification, mineral separation, and biological sensing. Spatial confinement down to the nanometer scale allows one to separate ions by their size, which, however, fails for ions with similar diameters of hydration. This selectivity can be boosted by enhancing the confinement to be comparable with or even lower than the size of hydrated ions, forcing the hydration shells to be distorted, even destroyed, or tuning the ion-wall interaction. We perform molecular simulations to explore ion transport processes across graphene nanochannels by exploring the end effects where both nanoconfinement and chemical functionalization are involved. We calculated the free-energy profiles that include the hopping barriers for dehydration/rehydration and adsorption/desorption of ions at the ends as well as the diffusivity of ions inside the nanochannel. A composite-channel model is then constructed for realistic membranes. The model and related parameters reported here allow us to quantitatively analyze the performance of nanochannel-embedded materials or devices, which conclude that, beyond subnanometer confinement that may be technically challenging for large-scale applications, edge engineering of the nanochannels by functional groups can significantly enhance the hopping-specific selectivity even if the diffusion-specific selectivity is gentle.