In a study published in Nature Nanotechnology, a research team led by Prof. TANG Zhiyong and Prof. LI Lianshan from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences developed a conceptual advanced hydration-layer-mediated sieving strategy. By leveraging dynamic angstrom-scale hydrapores constructed from monolayer covalent-organic frameworks (COFs), the team achieved ultrahigh monovalent cation separation, driven by short-range interactions mediated by water molecules, which is a mechanism traditionally overlooked in membrane science.
"Conventional sieving methods, which rely on size exclusion, electrostatic repulsion, or even complete dehydration energy, often struggle with the well-known trade-off between permeability and selectivity" said Prof. TANG, "In real-world conditions, ions are tightly wrapped in hydration layers. Previous studies seldom focused on the short-range interactions between these hydration layers. However, our work demonstrates that within the nanoconfinement of water molecular dimensions, these short-range interactions play a decisive role".
Hydrated ions were anchored at the pore rims of a COF monolayer to construct dynamic hydrapores with an effective size of approximately 0.8 nm. The core mechanism involves "merging" and "squeezing" of hydration layers to control ion transport: K+ ions, possessing flexible hydration layers, facilitate "merging" and attractive interactions, enabling near-frictionless transport; in contrast, Li+ ions, with rigid hydration layers, undergo "squeezing" and repulsion, effectively blocking their passage. Experimental results confirmed that the activation energy for K+ transport is as low as 5.5 kcal/mol, approaching that of K+ self-diffusion in bulk solution, indicating near-frictionless transport. Under a concentration gradient, the membrane achieved an exceptional K+/Li+ selectivity of 148 and a K+ permeance of 2×104 mol m-2 h-1, three orders of magnitude higher than the state-of-the-art membranes.
"The essence of our design lies in the 'merging' and 'squeezing' of the hydration layers", commented Prof. LI Lianshan, co-corresponding author of this work, "Our study proves that within water-molecule-scale confinement, the traditional assumption that like-charged ions always repel each other does not hold universally. By modulating the flexibility of hydration layers, we can guide ions toward either attraction or repulsion".
Dr. YANG Jinlei, the first author of this work and currently an associate professor at the University of Chinese Academy of Sciences (UCAS), highlighted the broader implications, "This insight provides entirely new strategies to address the more challenging problems in nanoporous membrane-based separations, particularly the discrimination of species with similar physicochemical properties, such as precise lithium extraction from high-salinity brines and rare earth element separation".
Schematic Diagram: The monovalent ion sieving through hydration layer discrimination. (Image by YANG Jinlei et al)
Contact: LI Lianshan
National Center for Nanoscience and Technology (NCNST)
E-mail: lils@nanoctr.cn
