Graphene oxide-cation interaction: Inter-layer spacing and zeta potential changes in response to various salt solutions

The objective of this study is to investigate the interaction between graphene oxide (GO) laminate and salt solutions. The well-aligned GO laminates are distributed on the top surface of a polyvinylidene fluoride-polyacrylic acid (PVDF-PAA) microporous layer via vacuum filtration. The PVDF-PAA layer is formed after being cast on a non-woven support and immersed in a water coagulation bath. We illustrate herein how the inter-layer spacing of the GO laminate membrane is varied due to contact with salt solutions or water, which results in tunable ionic sieving effect. It is assumed that the water permeation through the GO composite membrane follows a modified Hagen-Poiseuille's law. The GO inter-layer spacing is calculated, and this spacing is shrunken to 2.8 nm in the presence of MgSO₄ and enlarged to 6.1 nm with Na₂SO₄ from the initial 4.8 nm of water-swollen GO. The permeance values of the filtrate from Na-solutions are unexpectedly higher than those of pure water. The strong GO-Mg²⁺ interaction causes the surface zeta potential toward a neutral charge, which results in low rejections (<12%) for MgSO₄ and MgCl₂ solutions. The intermediate GO-Na⁺ interaction maintains the negative zeta potential of the GO layer, which readily rejects Na₂SO₄ (79%). The cations can modulate surface zeta potential and inter-layer spacing, which in turn directly impact the anion permeation and salt rejection behaviors. Overall, GO-salt interaction plays an important role in tailoring GO membrane characteristics. These GO-cation behaviors have potential applications in water filtration, desalination, molecular separation, and ion sensing and detection.