Engineering highly effective nanofibrous membranes to demulsify surfactant-stabilized oil-in-water emulsions

Demulsification of oil-in-water emulsions is challenging but significant for oily wastewater treatment, especially for surfactant-stabilized oil-in-water (S–O/W) emulsions. Electrospun nanofibrous membranes exhibit promising performance as demulsification medium but the underline mechanisms are still unclear. In this work, the effects of membrane pore size and nanofiber surface wettability on the demulsification of the S–O/W emulsions were investigated. Two polyvinylidene difluoride (PVDF) nanofibrous membranes with different nanofiber diameter (#P2: 200 nm and #P10: 1000 nm) were developed by electrospinning. The superhydrophilic membranes (#Dopa-P2 and #Dopa-P10) were obtained by coating polydopamine layers on both membranes #P2 and #P10. The silver nanoparticle deposition and subsequent fluorination were conducted on the surfaces of the polydopamine-activated membranes to prepare superhydrophobic nanofibrous membranes (#Ag–P2 and #Ag–P10). The superhydrophilic membrane #Dopa-P10 with a membrane pore size matching the oil droplets performed the best demulsification result. The oil droplets size increased up to 15 times (from 6.5 ± 1.0 μm to 98 ± 7 μm) after passing through #Dopa-P10. The underlying mechanism should be the superhydrophilic nanofibers possessed excellent affinity to the surfactants layer surrounding oil droplets, which could destroy the stable status and facilitate the coalescence of the oil droplets in S–O/W emulsions. The hydrophobic and superhydrophobic membrane #P10 and #Ag–P10 showed little coalescence effects as they exhibited a negative repulsive force to oil droplets. The physical cutting effect dominated the coalescence process when oil droplets passing through the membranes (#P2, #Dopa-P2, and #Ag–P2) with pore size much smaller than oil droplets. The various surface wettability didn't show any obvious difference on the coalescence results.