Functional Group Coverage as a Molecular Control Knob for Ion Transport in MXene Nanofluidic Channels

Ion transport in two-dimensional nanofluidic channels is fundamentally governed by interfacial water structure and ionic hydration, both of which are highly sensitive to surface chemistry. While extensive efforts have focused on the type of surface terminations in MXenes, the influence of functional group coverage density remains poorly understood. Here, using molecular dynamics simulations, we demonstrate that F-termination coverage in Ti 3 C 2 F x MXene nanochannels acts as an independent and powerful control parameter for ion transport. By systematically increasing the coverage from 25% to 100%, the transport of both cations and anions is suppressed to varying extents. Our results reveal that ion transport is jointly regulated by spatial confinement and interfacial interactions, which collectively reshape the hydration structures and hydrogen-bond networks under nanoscale confinement. At low coverage (≤25%), steric hindrance dominates, providing relatively weak constraints and highly dynamic hydrogen-bond networks, thereby enabling fast and nearly symmetric ion transport. As the coverage exceeds 25%, interfacial effects become increasingly significant, and ion transport is governed by the combined influence of steric and interfacial interactions. Notably, at 75% coverage, two distinct hydrogen-bond configurations emerge within the nanochannel, leading to a 2.6-fold difference in diffusion rates between cations and anions and maximizing transport asymmetry. These findings identify functional group coverage as a molecular “control knob” for tuning ion transport symmetry and selectivity in MXene nanofluidics.