Quantifying Effective Dehydrated Ion Sizes Based on Pore-Ion Steric Properties to Predict Separation Selectivity

Designing selective membranes for sieving specific solutes requires a deep understanding of fundamental structure-property-performance relationships, in which ion hydration properties under nanoconfined environment are the pivot for nanofiltration (NF) models and high-performance membrane synthesis. Herein, four nanochannels of similar components and structures but various sizes were constructed, and the transport manners of typical cations were tested for analyzing the effects of size-related dehydration process. Notably, dehydration extent reversed the ion transport rates in the nanochannels, while the trans-membrane energy barrier increased until a plateau was reached with the shrinkage of pore sizes, where the transformation from dehydration to deformation occurred in ion partitioning into the membrane pores as evidenced by theoretical calculations. Through quantitatively assessing sieving-related features of channels and cations, a correlation relationship between trans-membrane energy barriers and physical pore-ion parameters was obtained and then effective dehydrated sizes were calculated accordingly for replacing Stokes radius in diffusion description models. The diffusion rates linked to the effective radius were successfully proved to predict the separation ratio between alkali-metal ions, indicating the crucial role of hydration structure changes in controlling the ion trans-membrane process.