Relationship between desalination performance of graphene oxide membranes and edge functional groups

High desalination efficiency in principle could be achieved by layer-by-layer graphene oxide (GO) membranes, which benefits from their entrance-functionalized channels assembled by edge-functionalized GO nanosheets. The effects of these edge functional groups on desalination, however, are not fully understood yet. To study the isolated influence of three typical edge functional groups, namely, carboxyl (-COOH), hydroxyl (-OH), and hydrogen (-H), molecular dynamics simulation was used in this work. The results revealed that the edge volumetric blockage effect, resulting in ion permeability at G-H > G-OH > G-COOH membranes, was the dominant mechanistic effect inside the GO membranes with 7 Å interlayer channels. The OH edge has the same effect as the H edge in NaCl/water selectivity because of a unique "ion pulling" effect. Moreover, the OH and H edge-functionalized membranes with 7 Å interlayer channels showed preferential Na⁺ and Cl^- rejections, respectively. This kind of preference leads to a cycle of charging and neutralization in the penetrant reservoir throughout the filtration process. The results from this work suggested that it would be strategic to keep the COOH and H edge functional groups, to maintain the size of interlayer channels in order to stimulate the effects of edge functional groups, and to increase the membrane porosity for designing higher desalination efficiency GO membranes.