Improved Water/Polar Liquid Separation Performance of Graphene Oxide Membranes by Increasing the Starting Dispersion Concentration

Graphene oxide membranes (GOMs) are promising materials for separating water from polar organic liquids, but their separation performance depends strongly on membrane architecture and fabrication conditions. Three free-standing GOMs were prepared from the same starting graphene oxide dispersion at two concentrations: 2 mg mL⁻¹ (isotropic) and 10 mg mL⁻¹ (liquid-crystalline). The orientation of graphene oxide layers within the membranes was examined using spin-probe technique. For one membrane prepared from the concentrated dispersion, the nitroxide spin probe H2 was added before filtration; for the other two membranes, the probe was introduced after membrane formation. Lamellar orientation was evaluated by SEM image analysis. The second-rank lamellar order parameters were similar for all three membranes ( T 2 = 0.86–0.92), whereas the layer order parameters were lower ( P 2 = 0.49–0.62) and differed between membranes prepared from isotropic and liquid-crystalline dispersions. Water-vapor permeance was comparable across all membranes, at approximately 0.046–0.052 mol m⁻² s⁻¹ bar⁻¹. In contrast, ethanol- and acetonitrile-vapor permeances decreased by a factor of approximately 4 and 2.5, respectively, for membranes prepared from the concentrated dispersion. Consequently, their water/ethanol selectivity increased from 12 for the isotropic-dispersion membrane to 36–43 for the membranes prepared from the concentrated dispersion, while water/acetonitrile selectivity increased from 9 to 21. These results show that increasing the starting dispersion concentration can substantially improve transport selectivity while maintaining similar water permeance. The two membranes prepared from the concentrated dispersion showed similar permeance and selectivity.