Diffusion of Water Confined between Graphene Oxide Layers: Implications for Membrane Filtration

Among the carbon-based two-dimensional materials, graphene oxide (GO) has been attracting a growing interest due to its capability to be utilized in the field of water remediation. Therefore, an atomistic understanding of the transport properties of water in layered GO is pivotal for the development of novel GO membranes. Surprisingly, the very issue of the two-dimensional self-diffusion of water confined between two GO sheets appears to be controversial and simulations showing either a slow-down or no effect have been reported. In any case the formation of Hydrogen bonds, i.e. among the confined water and between water and the GO functional groups, was identified to control diffusion. However, results of molecular dynamics simulations heavily depend on the used forces. Density functional theory and empirical force fields are on opposite when it comes to accuracy and numerical costs. As a compromise in the present study we performed molecular dynamics simulations using a density functional theory-based tight method (xTB) to investigate the diffusion of water confined between GO sheets. Specifically, we considered six GO/water models, differing in the ordering of epoxide and hydroxyl groups as well as in the thickness of the water layer. For these models, having GO inter-layer distances between 8 and 12 \AA{} we find a reduction of the diffusion coefficient by a factor in between two and three as compared with bulk water. One possible origin of this effect is the temporary trapping of water within Hydrogen-bonded water bridges between the GO sheets. The proposed mechanism should be taken into account when developing, for instance, GO membranes for water remediation or applications in the field of selective transport in separation membranes.