Water’s motions in x–y and z directions of 2D nanochannels: Entirely different but tightly coupled

Two-dimensional (2D) material-based membrane separation has attracted increasing attention due to its promising performance compared with traditional membranes. However, in-depth understanding of water transportation behavior in such confined nanochannels is still lacking, which hinders the development of 2D nanosheets membranes. Herein, we investigated water confined in graphene or MoS₂ nanochannels by molecular dynamics (MD) simulations and found water’s diffusivity always varied linearly with their mean square displacement along z direction ( $⟨$Δz² $⟩$) when system variables (e.g., water molecules’ number, channel height, nonbonded interaction parameter, and harmonic potential constraining water’s z-coordinate) changed. Such linear correlation applies to different water models and different force fields (FFs) of channel walls (e.g., different Lennard–Jones parameters or even flexible FF), no matter whether water molecules form 3-, 2-, or quasi-2-layer structure in the nanochannel. This indicates, though water molecules’ motion along z direction (z-fluctuation, confined within 1 nm) and that in xy plane (xy-diffusion) are entirely different, they are tightly coupled: Violent z-fluctuation would produce more transient void to facilitate xy-diffusion, which is to the sharp contrary of bulk water, where motions in x, y, and z directions are symmetric, but independent. Our work could help design high performance 2D nanochannels and discover more novel principles in nano-fluidics and membrane separation fields.