Water-Mediated Ion Selectivity in 2D MXene Channels

Background
At the Ångström scale, water confined between 2D layers behaves unlike bulk water, governing ion transport in natural and engineered nanofluidic systems. Yet how confined water mediates selective ion transport remains poorly understood. Classical descriptions of aqueous ion transport break down under extreme confinement, and experiments struggle to control both nanoconfined structure and surface chemistry. This limits our ability to explain and predict water and ion behavior in subnanometer channels and membranes. Target: artificial ion channels, membrane separation, desalination, and lithium extraction.

Innovation
A Ti3C2Tx MXene platform with precisely controlled interlayer spacing (0.9–5.0 Å), surface terminations, and electrode potential enables systematic tuning of ion transport. Combining ion permeation, SIMS, FTIR, and MD simulations reveals that ultranarrow confinement reorganizes confined water, imposes ion-specific dehydration penalties, and modulates ion–MXene interactions. Li+ permeation in horizontally aligned channels is about two orders of magnitude faster than in conventional vertically aligned MXene membranes. Electrochemical surface charge modulation further regulates ion selectivity. Target: electrically gated nanofluidics and high-selectivity separation membranes.

Materials
The study focuses on Ti3C2Tx and Ti3C2Cl2 MXene nanosheets. Intercalation/hydration and physical constraint fix the d-spacing and align flakes horizontally to form nanochannels. The hydrophobic Cl-terminated surface suppresses water transport, pushing most cation permeation below detection limits and enhancing Li+ selectivity. Simulations show that diffusion and exit barriers dominate transport, while water reorientation tunes ion–surface coupling. Target: ion separation, desalination, ion recovery, and water treatment.

#MXene #2DMaterials #IonSelectivity #Nanofluidics #MembraneSeparation #Desalination #JACS #FrontierResearch