Grafting-free PVDF/PAN Janus membrane for high-flux water-gap membrane distillation and oil–in-water emulsion separation

This research work introduces a grafting-free Janus membrane suitable for desalination and oil-water separation applications. Two different polymers, namely PVDF and PAN, were used to fabricate the Janus membranes. The hydrophobic PVDF layer was made using a non-solvent induced phase separation (NIPS) process, while the hydrophilic PAN layer was electrospun onto the skin layer of PVDF by varying deposition time to generate asymmetric wettability without chemical modification. The fabricated membranes were characterized for surface morphology, elemental analysis, chemical and structural insights, bulk properties, and surface wettability. The optimized Janus membrane demonstrated significantly higher water flux of 32 kg m⁻² h⁻¹ with a salt rejection of > 99.99%, representing nearly a three-fold enhancement compared to the pristine PVDF membrane, which showed a flux of 10 kg m⁻² h⁻¹ and 98.75% salt rejection in water gap membrane distillation. Long-term operational stability under saline conditions, membrane wetting resistance, and specific thermal energy efficiency were also systematically evaluated. Furthermore, the optimized membrane also exhibited efficient oil-water emulsion separation, demonstrating a flux of 240.50 Lm⁻²h⁻¹bar⁻¹ with oil rejection of ∼99.99% in a dead-end setup, highlighting the multifunctional nature of the Janus architecture. Based on experimental results, the developed Janus membrane presents a multifunctional and sustainable platform, offering direction-dependent separation performance for both desalination and oil-water separation while eliminating chemical grafting, surface modification, and post-treatment steps. This work demonstrates that careful control of asymmetric wettability and interfacial adhesion alone can overcome the traditional flux-selectivity trade-off in PVDF-based membranes and provides a scalable route toward high-performance desalination membranes.