Controlled Nafion Modification of Membranes by Countercurrent Heating for Emulsion Separation

Janus membranes, exhibiting an asymmetric wettability and a thin functional layer, have attracted extensive attention for demulsifying oil-in-water emulsions and subsequently separating and recovering oil from them. But as yet, it is challenging to achieve facile and highly controlled strategies to construct asymmetricity and functionality within a very thin modified layer of Janus membranes. Herein, we report a controlled solution decoration via a restricted diffusion route induced by countercurrent heating to fabricate the Janus membrane with a thickness-controllable Nafion-modified layer. By applying a thermal field under the opposite side of a base membrane, the modifier (Nafion) solution can diffuse along the membrane pores with a confined diffusion distance because of the competition between the solvent infiltration induced by capillary force and the solvent evaporation driven by heating. Nafion can thus be painted on a hydrophobic membrane with a very low thickness (2–3 μm) that can be flexibly tuned by varying countercurrent heating temperatures and brush-painting cycles. Due to the amphiphilicity of perfluorosulfonic Nafion, the coated layer works as a hydrophobic surface in air while as a hydrophilic/oleophobic one underwater. The negatively charged sulfonic groups also endow the Janus membrane with a demulsification function. In this way, the Janus membrane can demulsify oil-in-water emulsions stabilized by cationic surfactants and then recover the free oil through unidirectional transportation across the membrane, maintaining a stable separation performance for at least five cycles to recover highly pure oil (oil purity >99.8%). Moreover, the Nafion-modified coating endows the prepared Janus membranes with enhanced thermal and chemical stabilities, with uncompromised wettability after immersing in strongly acidic and basic solutions for 120 h. This single-side surface painting method combined with countercurrent heating from the opposite side will pave the way for the practical applications of Janus membranes with high scalability.