Scalable and switchable CO2-responsive membranes with high wettability for separation of various oil/water systems

inspired by the capillary phenomenon in nature, we present a conceptually novel design strategy for the fabrication of CO2-responsive membranes through a capillary force-driven confinement self-assembling method.
Published in Materials
Scalable and switchable CO2-responsive membranes with high wettability for separation of various oil/water systems

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In 2018, when I first came into contact with membrane science in a lecture, I became interested in it and thought it was a science that could improve people's lives, because membrane can be used to purify water and air. In many parts of the world, there are serious pollution problems in the air and water, which is seriously damaging people's health.

Finally, in 2020, I was lucky to have an opportunity to join the membrane separation and membrane process (MSMP) research team of the School of Chemistry and Materials Engineering of Jiangnan University which is supervised by associate professor Liangliang Dong.I beacme a master student since that time, and officially started my scientific research career. The application of membrane separation in oil/water separation has become the first subject of my master's stage, we are looking forward to achieve separating complex oil/water mixture system with just one membrane. Therefore, we have a strong interest in controlling the surface properties of the membrane, because this is of great significance to the controllable separation of oil and water.

The traditional membrane has a single wettability, which limits it’s application to one of "water removal" or "oil removal", so it is difficult to achieve complete separation of complex oil/water mixture system. In recent years, switchable wettability oil/water separation membranes have been greatly developed. These membranes can perform structural, morphological or molecular conformation conversion in response to external stimuli (such as temperature, pH, electricity, light, magnetic field or ion) to adjust surface wettability and liquid transport channels, and realize controllable oil/water separation. However, these membranes often have complex and expensive manufacturing processes (such as chemical grafting and layer by layer self-assembly), which hinders their large-scale application, and also lead to a sparse or inhomogeneous presence of responsive moieties in the membrane, which is accompanied by inadequate responsiveness of surface wettability and deficient separating controllability of various emulsions. Moreover, there are significant limitations in the application of the above trigger factors, including economic and environmental costs and product contamination.

Dr. Dong has conducted many years of research on CO2-responsive polymers in his doctoral stage, and has achieved many great research results. After learning his work, CO2-responsive polymers are really surprising to me, they can change their properties under the stimulation of CO2 and inert gases. Compared with other stimuli, CO2 is non-toxic, cheap, and does not accumulate chemicals, it can be easily added to or removed from the system under operating conditions. We believe that CO2-responsive materials may achieve the goal of separating complex oil/water mixture system with just one membrane. In the process of achieving this work, we encountered the following difficulties: First, the current CO2-responsive membrane is only effective in separating immiscible oil/water mixture, which is difficult to achieve the separation of emulsified oil/water mixture; Second, because the manufacturing strategy of CO2-responsive membrane is similar to that of other stimulus response membranes, some key problems, including complex manufacturing, low productivity and difficulty in scaling up, are still inevitable, which means that the current manufacturing strategy is limited to laboratories and difficult to industrialize. Therefore, a facile and low-cost route to fabricate CO2-responsive membranes remains an elusive challenge for exploiting their application potential in complicated emulsion systems, and research efforts in this direction are critically needed.

Here, inspired by the capillary phenomenon in nature, we propose a conceptually novel design strategy for manufacturing CO2-responsive membranes by capillary force-driven constrained self-assembly (CFCS) method. This method is realized by parallel stacking two hydrophobic substrates with clearance to form capillary force, so as to drive the CO2-responsive polymer solution (poly(diethylaminoethyl methacrylate-co-methyl methacrylate, PMMA-co-PDEAEMA) into the confined area, and then self-assemble in situ on the surface and inside of the fabric as shown in Figure 1A. By manipulating the capillary force, PMMA-co-PDEAEMA can uniformly adhere to the fabric to enhance the switching ability of membrane surface wettability. Scanning electron microscope is used to characterize the morphological evolution of both sides, as shown in Figure 1B, when the gap width is from 150 μm increased to 300 μm, the fiber skeleton and 3D braided structure on side A are gradually covered by the copolymer, while this structural information can always be observed on side B. In addition, with the increase of gap width, the difference between WCA on both sides jumped from about 0 ° to about 24 ° (Figure 1B), which confirmed that large gap width was not conducive to the formation of uniform surface wettability on both sides. COMSOL simulation was carried out to further study the assembly behavior of copolymers on each side of the fabric substrate, and this information was used to clarify the morphological differences on both sides of the membrane (Figure 1C). In this work, we realized the effective separation of immiscible oil/water mixture and surfactant-stabilized emulsions, and more importantly, we realized the separation of multiphase emulsion mixtures, as shown in Figure 1D. The membrane first contacts the O/W phase emulsion, and allows the filtrate (i.e. water) to enter the water-containing region under the stimulation of CO2. After that, contact W/O phase emulsion. After CO2 is removed by N2 bubbling, the filtrate (i.e. oil) is allowed to enter the oil-containing region. The separation efficiency of each process can be stabilized at>99.5% (Figure 1E). Therefore, effective and continuous separation of multiphase emulsion mixture can be achieved by changing CO2/N2 stimulation. 

Fig. 1 Design of membrane and gas switchable multiphase emulsion mixtures separation. A Schematic illustration of the fabrication process of PPFM through the capillary force-driven confinement self-assembling method. B SEM images and WCA of the asprepared PPFM surfaces on two sides. The scale bar is 200 μm. C Velocity field of the copolymer solution on each side under different gap widths based on the COMSOL simulation. With increasing gap width, the velocity of the copolymer solution on the A side is larger than that on the B side, resulting in a larger accumulation of the PMMA-co-PDEAEMA copolymer on the A side. D Schematic showing the two-step separation process for the multiphase emulsion system under CO2/N2 stimulation. E Separation efficiency of PPFM-0.5 with gap width of 150 μm in the 1st and 2nd step operations. The emulsion content is oil content for O/W emulsion and water content for W/O emulsion, respectively.

More details can be found in our paper " Scalable and switchable CO2-responsive membranes with high wettability for separation of various oil/water systems" published in Nature Communications.

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