We also performed pressure driven filtration to study the intercalation stability. Deionized water was filtered through intercalated GOMs under ~0.9 bar of vacuum pressure for 5 h. The ICP-OES results of each water samples were listed in Table for X-M-GO and Table for X-S-GO. The detection limit of ICP-OES instrument is 10 µg/L. We found the elution of cations were highest in the first hour of filtration and rapidly fell in the second hour. The cation concentrations were undetectable in the 5 th hour for all samples. Previous studies suggest that cations intercalate GO mainly via cation-π interaction . It may also occur at sp 3 carbon areas, where the epoxy, hydroxy and carboxy groups exist. Such intercalation may affect the slip length when water molecules transport through GO membranes. The carboxy groups, located at the edges of GO sheets , have limited contribution to the morphology of the GO nanochannels. Therefore, the cation intercalation with carboxy groups may not affect the slip length. On the other hand, hydroxy and epoxy groups are located at basal plane of GO sheets which may affect the slip length. To investigate We observed that the water molecules are rearranged and become denser due to the attraction of positively charged cations. However, unlike the case of cation-π interaction (Figure ), the density of water molecules around the Na + , K + and Ca 2+ ions show no obvious difference in water affinities (Figure ). Based on that we believe that the cation intercalation with sp 3 carbon area cannot explain the trend of the slip length observed in Figure . As the water affinity shows a matching trend if the cations interact with the sp 2 carbon (Figure -E), it suggests that the cation-pi interaction is the more dominant case for governing the slip length.