Weakly hydrated ions can accelerate water transport through two-dimensional nanochannels by disrupting confined hydrogen-bond networks, according to a recent study of cation-exchanged vermiculite membranes.
Water confined at the nanoscale often moves faster than classical hydrodynamic models predict. Although this behavior is commonly studied in simplified ion-free systems, confined water in natural and engineered channels usually contains dissolved or surface-bound ions. The new study examined how those ions alter water structure and flow under two-dimensional confinement.
The researchers used vermiculite-based laminar membranes as a model system, exchanging the interlayer cations with lithium, sodium, potassium, cesium, or ammonium ions. Inductively-coupled plasma mass spectrometry confirmed comparable cation exchange across the membranes, while X-ray diffraction showed that hydrated interlayer spacing increased with cation hydration diameter.
Water transport, however, did not follow the same geometric trend. In evaporation and pressure-driven permeation experiments, membranes containing weakly hydrated cations showed faster water movement than those containing strongly hydrated cations. Potassium-exchanged membranes showed the fastest pressure-driven transport, despite having smaller hydrated interlayer spacing than lithium-exchanged membranes.
To connect that behavior to molecular structure, the team used infrared spectroscopy to probe the O-H stretching response of mobile confined water. After subtracting dry-membrane spectra, the confined-water signal showed cation-dependent shifts in the O-H stretching band. From lithium- to potassium-exchanged membranes, the band shifted toward higher wavenumbers, consistent with weaker hydrogen bonding and reduced energetic cost for water molecules to escape the network.
Further spectral analysis separated the O-H band into components associated with different hydrogen-bonding regimes. Strongly and intermediately hydrogen-bonded water accounted for most of the confined water population, while weakly hydrated cations favored less connected hydrogen-bond configurations. The authors describe this as “direct spectroscopic evidence” that ion-specific Hofmeister-type effects persist under nanoconfinement.
Machine learning-accelerated molecular dynamics simulations supported that interpretation. Lithium ions stabilized slowly diffusing hydration and interfacial water near the vermiculite surface, whereas potassium ions weakened local hydrogen bonds, shortened hydrogen-bond lifetimes, and increased the population of dynamically exchanging hydration water. That exchanging population dominated in-plane diffusion under confinement.
The study suggests that ion identity can influence confined water transport by reshaping the hydrogen-bond network inside nanochannels, not only by altering channel spacing. The authors argue that ion hydration should therefore be considered alongside pore size and surface charge when developing membrane and nanofluidic systems.
Newsletters
Receive the latest analytical science news, personalities, education, and career development – weekly to your inbox.
