Research Article www.acsami.org
Janus Graphene Oxide Sponges for High-Purity Fast Separation of Both Water-in-Oil and Oil-in-Water Emulsions Jongju Yun,†,∥,⊥ Fakhre Alam Khan,‡,⊥ and Seunghyun Baik*,‡,§ †
Department of Energy Science and ‡School of Mechanical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea
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S Supporting Information *
ABSTRACT: Membrane separation of oil and water with high purity and high permeability is of great interest in environmental and industrial processes. However, membranes with fixed wettability can separate only one type of surfactant-stabilized emulsion (water-in-oil or oil-in-water). Here, we report on Janus graphene oxide (J-GO) sponges for high purity and high permeability separation of both water-in-oil and oil-in-water emulsions. Millimeter-scale reduced GO sponges with a controlled pore size (11.2 or 94.1 μm) are synthesized by freeze drying, and the wettability is further controlled by fluorine (hydrophobic/oleophilic in air) or oxygen (hydrophilic/oleophilic in air) functionalization. J-GO sponges are prepared by the fluorine functionalization on one side and oxygen functionalization on the other side. Interestingly, the oil wettability of oxygen-functionalized surface turns into an oleophobic surface when immersed in water, which is explained by Young’s theory. This effect is further used in the separation of both water-in-oil and oil-in-water emulsions by changing the flow direction. The purity of the separated oil and water is very high (≥99.2%), and the permeability is more than an order of magnitude greater than those of the other Janus membranes reported. JGO sponges can be reused with an excellent repeatability, demonstrating feasibility in practical applications. KEYWORDS: graphene oxide sponge, Janus membrane, water−oil emulsion separation, high purity, high permeability by surface functionalization or polymer coating.9−13 The joined presence of hydrophobic and hydrophilic sides enabled the separation of both water-in-oil and oil-in-water emulsions using the same Janus membrane by changing the flow direction. The previously reported Janus membranes were made using cotton fabric or Kleenex hard roll towel.10,11 Optical imaging analysis demonstrated that both large oil droplets in water and water droplets in oil could be separated by changing the flow direction.10,11 However, the authors did not investigate surfactant-stabilized emulsions, and the separation purity was not provided.10,11 The pore size was also relatively large (100− 380 μm) compared with the typical emulsion droplet size (
