Controlled Nafion Modification of Membranes by Countercurrent Heating for Emulsion Separation

Abstract

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 Nafionmodified 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.

In Situ Doping of the PEDOT Top Electrode for All-Solution-Processed Semitransparent Organic Solar Cells

“…The functionality of F-acid was explored through density functional theory (DFT) calculations. The highly electronegative fluorine surrounding the PFA chain provides environmental inertness against both water (hydrophobic) and oil (oleophobic), possessing similar characteristics to the industrial fluoropolymer Teflon. , Moreover, the electrostatic potential (ESP) map presented in Figure C shows strong molecular polarization along the PFA chain with a dipole moment of 23.2 D compared with other sulfonate dopants (see Figure S3 and Table S2). Despite its lower acidity than H 2 SO 4 , strong polarization separates the electrostatic bond between PEDOT and PSS by aligning the dipoles in the opposite direction to those in PEDOT:PSS.…”

Polyethylenimine-Modified Glass Fiber by a Sulfur-Based Coupling Reaction for Selective Demulsification of Oil/Water Emulsions

“…However, it is challenging for these approaches to separate emulsified oil stabilized by a surfactant. Separation membranes with superwettability have unparalleled advantages for separating surfactant-stabilized emulsions, such as high efficiency, facile operation, energy saving, and marginal secondary pollution. − The critical step in the emulsion separation process is demulsification through the membranes.…”