Unveiling the transport of water and ions in the nanocage of zeolitic imidazolate frameworks by molecular dynamics

Precise ion/water and ion/ion separation requires pores of size closely aligned with the dimension of filtering molecules or ions. Zeolitic imidazolate frameworks (ZIFs), a class of porous crystallizes with well-defined Angstrom pores and diverse structures, making them promising candidates for nanofluidic channels. In this work, we perform molecular simulations to explore the structures and kinetics of water and ions confined in 4 typical ZIFs, including ZIF-65, ZIF-8, SALEM-2 and ZIF-Cl. Our findings reveal that the structure of water molecules within these ZIFs closely resembles that of bulk water but two diffusion regimes are identified. Notably, the diffusion coefficients (D) of the water are one order of magnitude lower than bulk water while the order of D for different ZIFs cannot be simply attributed to steric effects. The ion diffusion barriers exhibit a distinct contrast between monovalent and divalent cations, closely mirroring ion-valence selectivity as biological channels. Additionally, the Angstrom-scale pores within ZIFs demonstrate exceptional capability of filtering alkali metal ions with the selectivity as high as 10-100. Further analysis underscores the substantial effects of pore flexibility. The comprehensive simulation results strongly indicate that ZIFs hold significant promise for utilization in development of artificial ion channels and precise ion separation membranes.