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• The anion plays a crucial function in controlling the GO interlayer spacing. • The ion bridge is chiefly responsible for the reducing of the GO interlayer spacing. • The ion bridge is composed of one anion sandwiched by two cations. • The effect of the number of ion bridges on the GO interlayer spacing is investigated. Interlayer spacing of graphene oxide (GO) membranes can be precisely controlled using cations, which has been extensively used in nanofiltration membrane. However, no report definitely discloses the specific performance of anion in treating GO membrane, although cations and anions naturally coexist in aqueous solution. Subjectively, anion might have weak/reverse effect in maintaining the interlayer of negatively charged GO, which is yet confirmed by no evidence. In this work, combining molecular dynamics simulations and density functional theory calculations, we have discovered a long-time unexplored effect that anion can also sustain the GO membrane interlayer — specifically, two types of ion bridges (confined within two neighboring GO sheets), one composed of one anion intercalated into two cations and the other composed of two anions sandwiched by two cations, can effectively and stably fix the interlayer spacing of GO membrane. Our calculations revealed that the ion bridges greatly enhance the interaction between two neighboring GO sheets, making them highly stabilize in aqueous solution. Moreover, free energy calculations and density functional theory calculations further confirm the critical role of ion bridge in controlling the interlayer spacing of GO membrane. Our findings highlight, for the first time, the undiscovered control of GO membrane interlayer with anions via forming an ingenious ion bridge, rather than the preconceived weak/reverse effect of anions, which could inspire the future application of anions (and ion bridge) in membrane design. [ABSTRACT FROM AUTHOR]