Nanostructured hydrophilic interfaces of PVDF Janus membranes for high salinity water treatment: A combined theoretical and experimental analysis

Membrane distillation (MD) performance is strongly governed by membrane surface properties, particularly the stability and functionality of asymmetric wettability interfaces. In this study, polyvinylidene fluoride (PVDF) membranes were fabricated via phase inversion on sandpaper and glass substrates, followed by the formation of an ultrathin, physically deposited nanolayer on the permeate facing skin surface to induce hydrophilicity while preserving the intrinsic hydrophobic support. Theoretical analysis of structural optimization and energetic properties based on density functional theory revealed that the Zn–PVDF interaction is the most thermodynamically favorable configuration, leading to the exposure of oxygen atoms on the outer surface and consequently enhancing the hydrophilicity of the membrane. This solvent-free surface engineering approach produced delamination-free Janus membranes with a water contact angle (WCA) gradient of up to 50°, enabling robust interfacial adhesion and long-term stability under high-salinity conditions. The nanolayer deposition minimally affected membrane porosity, thickness, and tortuosity but significantly improved surface roughness, wettability, and MD performance. In water gap membrane distillation (WGMD) using 70 g L⁻¹ NaCl feed, the optimal membrane achieved a flux of 38.31 kg m⁻² h⁻¹, which is 93% higher than pristine PVDF, while maintaining >99.99% salt rejection. Long-term tests with real seawater confirmed exceptional durability, wetting resistance, and the ability to preconcentrate brine for membrane distillation crystallization and mineral recovery. This study demonstrates that precise control over nanolayer formation enables the design of structurally stable Janus membranes, providing a sustainable and scalable route toward next-generation MD systems for high-salinity desalination.