Spider silk bioinspired superhydrophilic nanofibrous membrane for efficient oil/water separation of nanoemulsions

Membrane separation has been acknowledged as an effective strategy to treat oily emulsions based on size-sieving. For separating nanoemulsions, smaller membrane pores are required to reject nano-sized oil droplets; however, the permeate flux would be significantly restrained. Herein, bioinspired by spider silk, a novel superhydrophilic electrospun nanofibrous membrane with spindle-knotted structures (SK-ENM) was constructed for highly efficient separation of nanoemulsions. The special spindle-knotted structures were constructed with 3–3.5 wt% polyacrylonitrile utilizing electrospinning based on Rayleigh instability. The prepared SK-ENM possessed a highly porous structure with a mean pore size of 0.634 μm, and strong superhydrophilicity/underwater superoleophobicity with rich micro/nano-hierarchical structures. For separating the surfactant-stabilized paraffin-in-water nanoemulsion with a mean droplet size of 325.7 nm, the oil rejection, permeate flux, and flux recovery rate were 95.68%, 1143.3 LMH/bar, and 99.98%, respectively. Thus, the SK-ENM with larger pores achieved both high oil rejection and high permeate flux for separating nano-sized oil droplets, which is a breakthrough achievement in overcoming the limitations of size-sieving. This could be attributed to the directional migration and coalescence of nano-sized oil droplets on spindle-knotted structures driven by Laplace force, which was calculated to be 8.7 × 10-8N. Meanwhile, the resulting large oil droplets were proved to be rejected by the superhydrophilic nanofibrous membrane with negligible fouling. This study provides insights into the design of novel functional superhydrophilic membranes for separation of emulsions. The as-prepared SK-ENM can be effectively applied to treating various nanoemulsions and complex oily wastewater in practical.