UCAS Team Unveils Hydration-Layer-Mediated Near-Frictionless Ion Sieving in Nature Nanotechnology

Associate Professor Yang Jinlei of the School of Nanoscience and Engineering at the University of Chinese Academy of Sciences (UCAS), together with researchers Tang Zhiyong and Li Lianshan of the National Center for Nanoscience and Technology, has published a paper in Nature Nanotechnology titled "Hydration layer mediates near-frictionless sieving of monovalent ions". By constructing dynamic angstrom-scale "water pores" and exploiting solvent-molecule-mediated short-range interactions, the work achieves high-selectivity, high-flux separation of monovalent cations, offering a new paradigm for overcoming the long-standing permeability-selectivity trade-off of conventional nanoporous membranes.

Ion-separation membranes are central to lithium extraction from salt lakes, water treatment and resource recovery. Yet strategies based on size exclusion, electrostatic repulsion or ion dehydration energy typically trade permeation flux for selectivity, or fail in complex high-salinity environments. The team noted that in real solutions ions are not bare: each is tightly wrapped by a shell of water molecules — the hydration layer — whose short-range interactions have long been overlooked in membrane separation science.

In this work, the team anchored hydrated ions at the pore rim of a monolayer covalent organic framework (COF) membrane, creating dynamic "water pores" with an effective size of about 0.8 nm. Whether an ion crosses depends entirely on the flexibility of its hydration layer:

• Potassium (K+) has a flexible hydration layer that fuses with neighbouring hydration shells near the pore, generating strong mutual attraction and near-frictionless ultrafast transport.
• Lithium (Li+) has a rigid hydration layer that is squeezed in confinement, generating strong repulsion that effectively blocks it outside the pore.

Measurements show the activation energy for K+ transport is just 5.5 kcal/mol, close to its intrinsic diffusion activation energy in aqueous solution. Driven by a concentration gradient, the membrane delivers a K+/Li+ selectivity of 148 and a K+ permeability exceeding 2×10⁴ mol·m⁻²·h⁻¹ — three orders of magnitude higher than the most advanced separation membranes reported internationally.

"What excites me most is that we broke the conventional physical assumption that ions of the same charge must repel each other," Yang said. At the sub-nanometre scale of water molecules, tuning hydration-layer flexibility can actively steer ions toward attraction or sustain repulsion, giving membrane design a fresh handle on harder nanoporous separation problems.

The team expects the approach to matter in precise lithium extraction from high-salinity brines, efficient separation of rare-earth elements, and the treatment of complex high-salinity industrial wastewater — scenarios where the species to be separated have near-identical physicochemical properties.