Steric Hindrance-Induced Dehydration Promotes Cation Selectivity in Trans-Subnanochannel Transport

Dehydration is a basic phenomenon in ion transport throughconfinednanochannels, but how it affects ion trans-membrane selectivity hasnot been understood due to a lack of characterization techniques andsuitable pore structures. Herein, hydration number distributions oftypical alkali metal ions were characterized by combining uniformsubnanochannels of ZIF-8-based membranes with the in situ liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS)technique, revealing that steric hindrance induced ion dehydrationthrough neutral confined ZIF-8 windows. The reduction in size dueto partial dehydration increased the intrapore velocity for monovalentcations. The highest entropy value with maximum size changes resultingfrom dehydration drove fast and efficient selective transport of Li+ over other alkaline metal ions, leading to a Li+/Rb+ selectivity of 5.2. The dehydration at the entranceof membrane pores was shown to account for the majority of overallbarriers, being a dominant element for ion transport. High hydrationenergy (>1500 kJ/mol) hindered the dehydration and transport oftypicalalkaline earth metal ions, achieving ultrahigh monovalent/bivalentcation selectivity (& SIM;10(4)). These findings uncoverthe crucial role of dehydration energy barriers and size-based entropybarriers in ion selectivity of trans-subnanochannel transport, providingguidelines for designing selective membranes with specific pore sizesto promote the dehydration of desired solutes.

Automated summary

The hydration number distributions of alkali metal ions were characterized using ZIF-8-based membranes and the in situ liquid time-of-flight secondary ion mass spectrometry technique. It was found that ion dehydration was induced through neutral confined ZIF-8 windows, which increased the intrapore velocity for monovalent cations. The study also revealed the crucial role of dehydration energy barriers and size-based entropy barriers in ion selectivity of trans-subnanochannel transport.