Single-Step, Nanoparticle-Free, and Durable Omniphobic Modification of the Poly(vinylidene fluoride) Electrospun Membrane Surface toward Enhancing Membrane Distillation

Membrane distillation (MD) provides a promising solution to the worldwide water scarcity through desalination and purification of wastewaters regardless of concentration. However, long-term performance and, therefore, the acceptability of the MD technique are threatened by membrane wetting. Many efforts have been devoted to adressing this crisis through membrane surface modification. In this regard, the simplicity and durability of the modification technique are of paramount importance. In this study, an omniphobic membrane was developed via a single-step nanoparticle-free technique by electrospinning a fluorinated homopolymer, poly(1H,1H,2H,2H-perfluorooctyl acrylate) (PPFOA), on the surface of a poly(vinylidene fluoride) (PVDF) nanofibrous membrane. Actually, electrospinning a low-surface-energy polymer such as PPFOA makes it possible to simultaneously benefit from surface roughness and surface chemistry towards decreasing surface energy and hence developing an omniphobic surface. It was found that the best results are achieved once the thicknesses of both PVDF and PPFOA layers are optimized. The developed omniphobic membrane with optimized PVDF and PPFOA thicknesses showed outstanding resistance against wetting as evidenced by high water entry pressure (2.2 bar) and large contact angles of 136.5 and 126.3° measured, respectively, for engine oil and isopropanol, the well-known low surface tension liquids. Additionally, despite the pristine PVDF electrospun membrane, the omniphobic membrane exhibited stable air gap MD (AGMD) performance for at least 900 min in direct contact with 3.5 wt % NaCl aqueous solution containing 0.2 mM sodium dodecyl sulfate (SDS).To further evaluate the long-term performance of the optimized omniphobic membrane, the SDS concentration in the MD feed solution was progressively changed from 0.2 to 1.0 mM, and it was observed that the MD process was continued for 720 min with approximately constant flux and salt rejection. To examine the durability of the modification technique, after a long-term evaluation experiment conducted for 15 h, the contact angles of water, engine oil, and isopropanol on the surface of the omniphobic membrane were measured again, and no remarkable changes were recorded. Considering the simplicity and durability of the introduced modification technique, it can be considered a cost-effective and applicable technique to produce omniphobic membranes with low surface energy on a larger scale.