Effects of different surfactant properties on anti-wetting behaviours of an omniphobic membrane in membrane distillation

Surfactant-induced membrane wetting remains a severe challenge for MD membranes, even for omniphobic membranes. The anti-wetting characteristic of the omniphobic membrane engendered by surfactant-contained feed solutions have not been systematically studied. In this work, a robust omniphobic membrane #PVDF-F with a super liquid-repellent property was developed. It involved constructing a fluorinated re-entrant structure on a dopamine-coated polyvinylidene fluoride (PVDF) nanofibrous membrane. Four different ionic surfactants, including sodium lauryl sulfonate (SLS), sodium dodecyl sulfonate (SDS), dodecyl trimethyl ammonium bromide (DTAB) and cetyl trimethyl ammonium bromide (CTAB) were added into the 3.5 wt% NaCl feed solutions to evaluate the anti-wetting performances of as-developed omniphobic, control nanofibrous and commercial PVDF membranes. Compared to SLS, the SDS with a higher hydrophilic-lipophilic balance (HLB) value tends to stay in the feed solution and reduces the surface tension of feed solution, thereby accelerating membrane wetting. Besides, when the experimental dosage (E d) is above its theoretical critical micelle concentration (CMC) value, the excessive free surfactant molecules form micelles and instantly adsorb onto membrane pore walls, promoting both convection and diffusion toward the permeate side and accelerating membrane wetting. Nonetheless, the omniphobic membrane still exhibited the most stable MD performance when treating the surfactant-stabilized oil-in-water emulsion.