Molecular insights into H3O+/Fe2+ sieving on functionalized graphene surface: The role of the ionic microstructures caused by different oxygen groups

[Display omitted] •According to the results of the MD simulation, the density of Fe2+ and H3O+ near the functionalized graphene surfaces is significantly higher than bulk density, which affects the H3O+ permeability and H3O+/Fe2+ selectivity.•The functionalized graphene surface with xepoxy of 30 % can achieve high H3O+ permeability and H3O+/Fe2+ selectivity.•The microscopic mechanism of high H3O+ permeability is that epoxy groups lead to strong interfacial group-H3O+ hydrogen bonds and a larger effective hydration radius.•The microscopic mechanism of high H3O+/Fe2+ selectivity is that epoxy groups induce the large density ratio and the large effective hydration radius ratio of H3O+/Fe2+. In this study, we utilize a series of molecular dynamics simulations to explore the effects of microstructures of H3O+ and Fe2+ ions on their transport and sieving behavior on a functionalized graphene surface. The functionalization of the surface is modified by varying the epoxy group (i.e., xepoxy = epoxy/(epoxy + hydroxyl)) from 0 to 60 %. Results demonstrate that significantly high ion density, relative to the bulk, occurs within the range from the solid surface to the first valley of Fe2+, which is defined as the transport region (0 to 6.7–10.3 Å). For xepoxy values in 0–20 and 40–60 %, the high number of hydrogen bonds (H-bonds) between interfacial groups (namely, hydroxyl and epoxy groups) and H3O+ results in a high H3O+ density in the interfacial contact layer, subsequently leading to a high Fe2+ density in the subcontact layer. Conversely, xepoxy value of 30 % results in a relatively high H3O+ density and the lowest Fe2+ density in the subcontact layer. A detailed examination of the hydration microstructure reveals that the larger effective hydration radius induces the higher diffusion coefficient of H3O+ and Fe2+, which occurs because the larger effective hydration radius, which corresponding to weaker hydration ability, allows the easier exchange of hydrated water in the second hydration shell with non-hydrated water, thereby increasing the ion diffusion coefficient. High H3O+ permeability is shown to arise from the combined effect of interfacial H-bond structures and the large effective hydration radius. Additionally, the H-bond structure and ionic hydration microstructure drive high selectivity due to the induced high-density and effective hydration ratios, respectively. Notably, a xepoxy value of 30 % offers relatively high H3O+permeability and the highest H3