Confined in-situ photopolymerization of an ultra-thin zwitterionic hydrogel skin on polysulfone membrane assisted by PEI for robust and enhanced emulsion separation.

Hydrogel-coated membranes resist fouling in oil-water emulsion separation through surface hydrophilicity, yet suffer from high initial flux loss and poor long-term stability caused by thick coatings and weak non-covalent adhesion. A novel "hydrophobic initiator-pre-embedded and confined surface-initiated" (HIPECI) photopolymerization strategy is reported for grafting an ultrathin zwitterionic hydrogel skin onto a polyethylenimine (PEI)-modified polysulfone (PSF) membrane. UV activation of a pre-embedded benzophenone initiator generates surface-tethered radicals for concurrent polymerization and covalent grafting of hydrogel skin. The PEI acts synergistically as a porogen and a multifunctional anchor, establishing covalent and noncovalent (electrostatic/hydrogen-bonding) linkages for robust interfacial adhesion. Consequently, the resulting membrane featured an ultrathin hydrogel skin (∼85 nm), delivering a > 2.5-fold higher emulsion flux and a > 50 % reduction in initial water flux attenuation rate compared with conventional hydrogel-coated membranes, while achieving >96 % oil rejection. It also showed robust stability against mechanical damage and extreme pH (1-13) for 168 h. This approach was successfully applied to recover high-value traditional Chinese medicine (TCM) volatile oils, achieving a high permeance of 1700 L m⁻² h⁻¹ bar⁻¹ and a rejection rate above 94 %. Overall, this work presents an effective strategy for developing antifouling, high-performance membranes for treating oily wastewater emulsions.