Molecular Dynamics Study of Water Flow Across Multiple Layers of Pristine, Oxidized, and Mixed Regions of Graphene Oxide: Effect of Graphene Oxide Layer-to-Layer Distance

Recent studies revealing exceptionally rapid water flow across graphene oxide membranes have highlighted them for potential filtration and separation applications. The physical and chemical features in graphene oxide membranes are heterogeneous, and there remains a great deal of speculation as to what is responsible for the facile water percolation. One potential contributing feature is the variation of interlayer spacing, which can occur naturally or be artificially induced. Herein, water flow across pristine, oxidized, and mixed membranes with interlayer distances of 0.7, 0.9, and 1.2 nm, corresponding respectively to the formation of discrete mono-, bi-, and trilayer water structures, was studied via molecular dynamics simulations. The interlayer spacing of 0.7 nm results in the formation of square ice for the pristine graphene membrane, which leads to collective motion, inhibiting equilibrium transport but allowing for rapid nonequilibrium flow comparable to that in the membranes with larger interlayer distances. A four-point time correlation function analysis of water structural relaxation reveals that collective water motions are responsible for rapid nonequilibrium flow for the interlayer spacing of 0.7 nm. Meanwhile, the central water layers formed in an interlayer spacing of 1.2 nm lead to almost entirely decoupled structure and dynamics between outer water layers.