Dual pH- and ammonia-vapor-responsive electrospun nanofibrous membranes for oil-water separations

In the last few years, the amount of industrial oily wastewater discharge and number of oil spill accidents have increased exponentially, spurring the development of highly efficient, cost-effective oil-water separation technologies. In this study, a dual pH- and ammonia-vapor-responsive polyimide (PI)-based nanofibrous membrane with high permeate flux and stability was developed for oil-water separations. The membrane was prepared by successively dip-coating electrospun PI in decanoic acid (DA)-TiO₂ and silica nanoparticles (SNPs). The novel SNP/DA-TiO₂/PI membrane exhibits superhydrophobicity in air and superoleophilicity in neutral aqueous environments (e.g., at pH 6.5). However, the membrane becomes hydrophilic and superoleophobic in basic aqueous environments (e.g., at pH 12), resulting in only water permeation during oil-water separations. The oil-water separation potential of this innovative dual-responsive membrane was investigated using several model oil-water mixtures. The membrane has extremely high flux (6500±100 L m⁻² h⁻¹) and separation efficiency (>99%) and is reusable. In addition, thermal stability and abrasion resistance tests show that the membrane is highly stable under extreme conditions. This dual-responsive electrospun nanofibrous membrane has potential for use in industrial oil-polluted water and oil spill treatment and in systems that require selective oil-water permeation. Furthermore, changes in the surface wettability can also be induced by exposure to ammonia vapor, which might facilitate remote-controlled oil-water separations.