Membrane wettability manipulation via mixed-dimensional heterostructured surface towards highly efficient oil-in-water emulsion separation

Membranes with super-wetting surface work well at separating oil-water mixtures, but efficient and continuous separation of surfactant-stabilized oil-in-water emulsion is difficult to achieve due to widely spread oil droplet size and stable droplet nature. This work reported a mixed-dimensional heterostructured oil-water separation membrane with electrospun hydrophobic polydimethylsiloxane/polyvinylidene fluoride (PDMS/PVDF) nanofibers on top of a hydrophilic polyaniline (PANI) coated microfiltration membrane support. It is discovered that small oil droplets are captured and coalesced by the hydrophobic-oleophilic PDMS/PVDF nanofibers, while water passes through the membrane from uncovered hydrophilic PANI micro-regions. Precise regulation on the proportion of hydrophilic and hydrophobic regions is achieved by controlling the electrospinning conditions, and highly efficient oil-in-water emulsion separation is thus accomplished with membrane pore size much larger than emulsified oil droplets. Multi-cycled and long-term crossflow separation experiments result in relatively stable membrane flux and oil-water separation efficiency higher than 99%, as coalesced larger oil droplets are continuously removed from the membrane surface by shear flow. Such heterostructured membranes with highly tunable wettability could therefore combine the merit of good separation efficiency and operation stability for practical surfactant-stabilized oil-in-water emulsion separation.