Deciphering wetting mechanisms in janus membranes via real-time impedance spectroscopy for stable oily wastewater desalination

The treatment of surfactant-stabilized oily wastewater poses a significant challenge to aquatic ecosystems and water reuse efforts. Thermal membrane desalination often suffers from membrane wetting and performance trade-offs when dealing with low-surface-tension effluents. In this study, a high-performance Janus membrane that exhibits exceptional anti-wetting properties has been developed. A key mechanistic insight is that membrane wetting is governed not merely by reduced surface tension, but critically by the hydrophobic-hydrophobic interactions between contaminants and the membrane. It is demonstrated that wetting requires both exposed hydrophobic segments and sufficient hydrocarbon chain length for effective engagement, explaining the distinct wetting behaviors of single-headed amphiphiles, bola-amphiphiles, and polymers. By employing real-time electrochemical impedance spectroscopy (EIS), the progression of wetting fronts is quantitatively elucidated, revealing how the membrane maintains stable states—such as light-penetration, intermediate-through, or deep wetting—under different chemical challenges without failure. The Janus membrane demonstrates superior vapor flux while maintaining salt rejection exceeding 99.9 %, attributed to its sub-nanoporous structure and synergistic mass transfer enhancement. Even under extreme surfactant concentrations (>20 mM), the membrane effectively resists wetting. Long-term evaluation using real oily wastewater over 30 days confirms unparalleled stability in both flux and rejection. The membrane's robust performance, combined with a straightforward and scalable fabrication method, offers a promising solution for the treatment of complex industrial wastewaters.