Molecular insights into multilayer 18-crown-6-like graphene nanopores for K⁺/Na⁺ separation: A molecular dynamics study

Currently, 18-crown-6-like graphene nanopore has emerged and received much theoretical and experimental attention. Inspired by the multilayer oxygen-containing structure of the K⁺ channel (KcsA), we adopted molecular dynamics to investigate the selectivity of K⁺/Na⁺ for a series of multilayer 18-crown-6-like graphene nanopores. The results demonstrated that the nanopores of the multilayer graphene could facilitate the selectivity of K⁺/Na⁺ compared to the monolayer graphene. A proper combination of layer number and interlayer spacing could achieve a high selectivity. The spatial distribution of fluid molecules and ionic hydration microstructures indicated that the easier dehydration of K⁺ could lead to the more uniformity of K⁺ pathways in the central region of the nanopore and in turn be helpful in increasing the selectivity of K⁺. Meanwhile, the increased unsuitability for the pore wall oxygen atoms to compensate the partial dehydrated Na⁺ could enhance the resistance of Na⁺. These findings provide useful insights to the further design of 18-crown-6-like and other crown-ether-like graphene nanopore-based nanodevices as sensors and ion separators.