A modified TA-APTES coating: Endowing porous membranes with uniform, durable superhydrophilicity and outstanding anti-crude oil-adhesion property via one-step process

Tannic acid (TA)-aminopropyltriethoxysilane (APTES) coating has excellent hydrophilicity, and can be applied to a variety of materials. But it has poor stability in the air. To improve its stability, we have developed TA-APTES-Fe(III) coating in our previous work, but there are still some problems remained such as tedious steps, and undesirable anti-crude oil-adhesion performance, etc. Besides, both TA-APTES and TA-APTES-Fe(III) coating cannot realize the hydrophilic modification of the inner surfaces of porous polypropylene (PP) membrane. To solve the above mentioned problems, we developed superhydrophilic and stable TA-APTES- tetraethylorthosilicate (TEOS) coating via one-step process in this study. The new coating can realize the superhydrophilic modification of the inner surfaces of porous PP membrane, and endows the PP membrane with good water permeability, which cannot be realized by our previously reported TA-APTES and TA-APTES-Fe(III) coating. Besides, the stability of the coating is greatly improved by introducing stable alcoholic hydroxyl groups derived from TEOS, and the resultant modified membrane can keep superhydrophilic (CA about 0°) and underwater superhydrophobic (underwater oil CA above 150°) even after being stored in air for more than 6 months, ultrasonic treatment for 10 min, or being rinsed by water for 24 h (3000r/min), exhibiting outstanding stability. More importantly, the crude oil adhesion force of the new coating is as low as 4 μN even under the preload force of 300 μN, showing excellent anti-crude oil-adhesion property. The PP membrane modified by the new coating possesses ultrahigh water flux (10384 L m^-2 h^-1 bar^-1) and can realize highly efficient separation of oil-in-water emulsion (above 99%). This work is expected to accelerate the practical application of TA-APTES coating.