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 (Delta z(2)) 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 (xydiffusion) are entirely different, they are tightly coupled: Violent z-fluctuation would produce more transient void to facilitate xydiffusion, 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.

Automated summary

This study investigates the behavior of water confined in graphene or MoS2 nanochannels using molecular dynamics simulations. The results show that the diffusivity of water is linearly correlated with its mean square displacement along the z-direction, regardless of changes in system variables. This work is significant for designing high-performance 2D nanochannels and discovering novel principles in nano-fluidics and membrane separation fields.