Abstract
Graphene has been recognized as a potential material for selective membranes for desalination, because of its atomic thickness, high mechanical strength and chemical resistance. Despite of the promising outlook, little is known about the slip of water and ions along the graphene plane, which is essential to achieve high water permeation and ion selectivity. In this work, we aim to scrutinize the graphene-solution interfacial composition and dynamics. High-quality graphene is grown on copper by chemical vapor deposition and then transferred onto functionalized substrates. Considering the intrinsic ionic environment in seawater, KCl and NaCl are selected as the investigated electrolytes. An AFM is used to measure the DLVO forces, non-DLVO forces, and adhesion, revealing the interfacial properties. Importantly, velocity-dependent friction-force measurements, as a function of the applied load and solution composition, are performed and described by a shear-promoted thermally-activated slip model based on Erying’s transition state theory. We thus obtain a stress-activation length that characterizes the mobility of the surface-adsorbed hydrated ions under shear, which plays a key role in the microhydrodynamic processes. Our results show remarkable ion-specific effects. The findings of this work directly contribute to the understanding of water permeation and ion rejection mechanisms by graphene membranes.