Abstract
This study leverages the ancient craft of weaving to prepare membranes that can effectively treat oil/water mixtures, specifically challenging nanoemulsions. Drawing inspiration from the core-shell architecture of spider silk, we have engineered fibers, the fundamental building blocks for weaving membranes, that feature a mechanically robust core for tight weaving, coupled with a CO2-responsive shell that allows for on-demand wettability adjustments. Tightly weaving these fibers produces membranes with ideal pores, achieving over 99.6% separation efficiency for nanoemulsions with droplets as small as 20 nm. They offer high flux rates, on-demand self-cleaning, and can switch between sieving oil and water nanodroplets through simple CO2/N2 stimulation. Moreover, weaving can produce sufficiently large membranes (4800 cm2) to assemble a module that exhibits long-term stability and performance, surpassing state-of-the-art technologies for nanoemulsion separations, thus making industrial application a practical reality.
Weaving core-shell fibers into membranes allows precise wettability control and tailored pores to treat diverse nanoemulsions.
INTRODUCTION
Oil/water emulsions are prevalent across diverse industrial processes, from petrochemical ventures to cosmetic applications (1, 2). Their effective treatment is vital, both for optimizing industrial output and safeguarding our environment. For example, in oil exploitation, the quest to boost oil recovery requires large amounts of water and surfactants, inevitably producing vast oil/water emulsions in wastewater (3). Addressing these emulsions not only mitigates potential ecological threats, such as disruptions to aquatic and marine ecosystems (4), but also facilitates the recovery of water and valuable solutes, like lithium commonly found in produced water, from waste streams.
Membranes have emerged as the preferred solution for treating emulsified oil/water mixtures, especially when compared to traditional methods like solvent extraction, centrifugation, flocculation, adsorption, and oxidation, which often suffer from inefficiencies or high costs (5, 6). However, the effectiveness of membranes is challenged when separating complex oil-in-water (O/W) and water-in-oil (W/O) nanoemulsions. These thermodynamically stable nanoemulsions, characterized by sub–200 nm droplets, require precise control over membrane attributes such as pore size, density, and wettability (6–11). Current membranes often underperform in real-world industrial conditions containing nanoemulsions due to difficulty in sieving nanodroplets and irreversible fouling. The ultimate goal is to develop scalable membranes with a high density of nanometer-sized pores, a narrow pore size distribution, and adaptable wettability to effectively address both O/W and W/O nanoemulsions and mitigate irreversible fouling.
We report a simple coating strategy to fabricate CO2-responsive core-shell fibers that can change their wetting properties on-demand. The fibers uniquely merge a CO2-responsive shell with a mechanically robust polyester core. Adopting scalable advanced industrial weaving techniques, we transformed these fibers into superwetting membranes that demonstrate a homogeneous distribution of CO2-responsive units both internally (e.g., within pore walls) and externally on their surface. Our weaving approach ensures a remarkably narrower pore size distribution compared to conventional methods, essential for the efficient separation of nanoemulsions. This design strategy provides unparalleled control over pore structure and facilitates dynamic on-demand adjustments of surface wettability, addressing the inherent limitations of current state-of-the-art nanoemulsion separation membranes. Through a systematic investigation of the fiber morphology, physicochemical properties, and the resultant membranes pore structure, surface wettability, and emulsion separation performance, we demonstrate the membrane potential for industrial applications. We further present a successful pilot-scale demonstration by incorporating a large-area CO2-responsive superwetting membrane into a standard commercial membrane unit. Overall, our work demonstrates resilient, smart separation membranes tailored for industrially relevant oil/water emulsion separations.
RESULTS
Fabrication of core-shell structure of CO2-responsive fibers
The natural mechanism in plants that adjust to carbon dioxide levels (i.e., CO2-responsive stomatal movements) (12–16) has inspired the development of CO2-responsive superwetting membranes for oil/water separation (17–19). When stimulated by CO2, these membranes can transform their physicochemical properties (e.g., pore structure and surface wettability) to regulate the permeability and separation performance, allowing for efficient separation of different oil/water mixtures on demand (20–23). Two prevailing strategies have been extensively adopted for crafting CO2-responsive superwetting membranes. The first involves the introduction of CO2-responsive moieties—linear polymer chains, cross-linked hydrogel networks, and microspheres—as modifiers. They are integrated into the surface or the pore walls of existing porous membranes through grafting or surface coating (22–27). The second method involves the direct usage of CO2-responsive polymers as raw materials or additives during membrane formation (20, 21, 28, 29). However, both strategies present notable drawbacks. The former often suffers from a sparse presence of responsive moieties in the membrane caused by a low grafting density, or issues related to poor coating cohesion causing pore clogging and shedding of the coating. The latter method, meanwhile, struggles with challenges including poor embedding of CO2-responsive moieties during membrane formation, loss of CO2-responsive moieties (especially for CO2-responsive additives) during the operation, and a broad pore size distribution in the resulting membranes. Consequently, membranes made using these methods are only suitable for immiscible oil/water mixtures and emulsified micrometer-sized oil/water mixtures and fail when challenged with nanometer-sized oil/water emulsions (i.e., nanoemulsions).
A potential solution might reside in the realm of core-shell materials that allow for enhanced flexibility and tailored functionalities (30–32). One can look to natural spider silks for a prototypical example of core-shell construction. These silks consist of a central core of nanofibrils encased within a thin plastic shell (as depicted in Fig. 1A) (33–37). Such a specialized core-shell microstructure affords substantial flexibility and enables the tailoring of surface functionalities to suit an array of application requisites. In this study, we engineered CO2-responsive core-shell fibers, combining a robust core ideal for crafting tightly woven membranes with small pore sizes and a CO2-responsive shell for precise on-demand wettability adjustments. Such fibers were fabricated using a simple one-step coating process using a sizing machine (Fig. 1B and fig. S1). Specifically, a bare polyester fiber was first dried at 80°C and then continuously pulled in and out a poly(methyl methacrylate) (PMMA)–co–poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) copolymer solution. As the solvent evaporates, the copolymer coating solidifies to form the core-shell fiber, which is finally collected onto a bobbin (Fig. 1C). A servo motor can be used to control the drawing speed making the process easy to automate.
Fig. 1. Design of CO2-responsive fibers with core-shell structure.
(A) Schematic showing the core-shell structure of spider silk. (B and C) Schematic of the one-step coating process to fabricate CO2-responsive core-shell fibers. (D and E) Surface and cross-sectional SEM images of polyester fiber before (D) and after four PMMA-co-PDEAEMA coatings (E) to construct the CO2-responsive core-shell structure. (F) Energy-dispersive x-ray mappings of the CO2-responsive core-shell fiber, where Fe was used to stain the PMMA-co-PDEAEMA shell. (G to I) Surface roughness of CO2-responsive core-shell fibers under different stimulations. Insets are the corresponding 2D AFM images with a scale bar of 1.0 μm. Whenever not specified, the number of PMMA-co-PDEAEMA coatings for the presented samples was four.
The core-shell structure was confirmed using scanning electron microscopy (SEM), as depicted in Fig. 1 (D to F) and figs. S3 and S4. In these images, the polyester fiber is enveloped by the copolymer with no discernible gap. By manipulating the number of coatings, the shell thickness of the resultant fiber can be precisely tuned, increasing from 117 to 178 μm as coatings progress from one to four (fig. S5). Increasing the number of coatings also improved the fiber’s mechanical properties, which is conducive to producing membranes with superior mechanical properties (fig. S6).
The CO2 responsiveness of the fibers was investigated by proving the surface roughness under different stimuli with atomic force microscopy (AFM; Fig. 1, G to I, and fig. S7). In the initial state, the surface of the core-shell fiber was smooth, evidenced by negligible height fluctuations (Fig. 1G). After bubbling CO2 for 10 min, considerable height fluctuations were measured, revealing a rougher core-shell fiber surface (Fig. 1H). Replacing CO2 with N2 recovered the surface to its initial state (i.e., a smooth surface; Fig. 1I). On the contrary, the surface of the blank fiber always remained smooth with negligible height fluctuation regardless of the stimuli (Fig. 1, G to I). On the basis of these findings, it is evident that the core-shell fiber has a robust gas-responsive surface. This establishes a solid groundwork for crafting CO2-responsive superwetting membranes with it.
Fabrication of CO2-responsive superwetting membrane
CO2-responsive superwetting membranes were fabricated by weaving the core-shell fibers using an industrial rapier loom (Fig. 2A and fig. S8). The morphology and pore structure of the membranes can be easily tuned by adjusting the weaving density, which changes the distance between the weft-warp contact points. In this work, we used the optimized core-shell fiber with four coatings to weave the CO2-responsive superwetting membranes. Three distinct weaving densities—low, medium, and high—were used in fabricating CO2-responsive superwetting membranes. These have been designated as CO2-responsive superwetting membrane (RSM)–L, CO2-RSM-M, and CO2-RSM-H, respectively. Control membranes were also fabricated with bare polyester fibers and the same weaving conditions and designated as blank-L, blank-M, and blank-H, respectively. SEM images (Fig. 2B and figs. S9 and S10) illustrate the membranes’ varying morphologies and show the CO2-RSMs lack the gaps present in blank membranes due to the PMMA-co-PDEAEMA shell, consequently providing a narrower pore size distribution. Capillary flow porosimetry confirmed a reduction in pore size with increased weaving densities (Fig. 2D), revealing that, in comparison, CO2-RSM membranes exhibited a more restricted pore size distribution than blank membranes. Depending on the weaving density, the average pore size ranged from 0.35 to 0.2 μm for the former and from 2.0 to 1.2 μm for the latter. It is important to note that for membranes containing PMMA-co-PDEAEMA, the effective pore size, specifically, the pore size experienced by nanodroplets during operation, is considerably smaller due to the presence of a layer of confined water and nanoconfined water when the membrane is in its hydrophilic state (fig. S16, D to F). This layer is more firmly bound to the membrane surface than free water. Conversely, a reversed phenomenon occurs when the membrane transitions to its hydrophobic state (fig. S16, G to I).
Fig. 2. CO2-responsive superwetting membranes via weaving.
(A) Schematic of the mechanical weaving process to fabricate CO2-responsive superwetting membranes. (B and C) Surface SEM images of CO2-responsive superwetting membranes with low (B; CO2-RSM-L) and high (C; CO2-RSM-H) weaving densities. Red and yellow lines show the distance between five adjacent weft-warp contact points in the weft and warp directions, respectively. Insets show schematics of the weaving density. (D) Pore size distribution of blank membranes made of polyester fibers and CO2-responsive superwetting membranes with core-shell fibers across low, medium, and high weaving densities. (E) Force-distance adhesion curves for water and oil droplets in oil and water environments, comparing the blank membrane (black line) to the CO2-RSM-H membrane (red line) under CO2/N2 stimulation. The measurement consisted of four stages: (i) the droplet approaches the membrane surface, (ii) the droplet contacts the membrane surface under a fixed load of 40 μN, (iii) the droplet undergoes shape deformation after leaving the membrane surface due to adhesion force, and (iv) the droplet completely detaches from membrane surface. Insets show the shape of the water or oil droplets during each stage for both the blank membrane (frameless) and CO2-RSM-H membrane (red frame).
The surface wettability of the CO2-RSM membranes changes when exposed to CO2. This is due to the hydrophobic tertiary amine groups in the PMMA-co-PDEAEMA shell transforming into hydrophilic protonated amine groups. We verified this behavior by measuring the water contact angle (WCA) under alternating CO2/N2 stimulation. As shown in fig. S11, the WCAs of all CO2-RSM membranes in the initial state are above 145° and then decreased to 0° after being exposed to CO2 for 10 min. Once the CO2 is replaced by N2, the WCA values return to the initial state. The CO2-RSM-H membrane exhibited a remarkable WCA shift from 151° to 0°, achieving the desired transition between superhydrophobicity and superhydrophilicity, ideal for membranes designed to purify nanoemulsions. This gas-responsive behavior, absent in blank membranes, is further corroborated by roll-off angle measurements (fig. S12), affirming the CO2-RSM membranes’ exceptional gas-tunable anti-adhesion property through switching surface wettability.
Dynamic adhesion force measurements, along with low-field nuclear magnetic resonance (LF-NMR), and surface-enhanced Raman spectroscopy (SERS), were used to delve deeper into the oil and water repellency of the CO2-RSM membranes. Figure 2E illustrates a scenario wherein an oil droplet (5 μl) and a water droplet (5 μl) were, in turn, pressed against the membrane surface with a predefined load before being allowed to relax. A consistent gas-tunable adhesion behavior was observed across all CO2-RSM membranes, maintaining the sphericity of oil droplets under water (CO2 stimulation) and water droplets under oil (N2 stimulation) throughout the entire advancing and receding process. The real-time force-distance curves, as depicted in Fig. 2E and figs. S13 and S14, reflect an ultralow adhesion force to both oil and water droplets, even at a high load of 120 μN, in particular in densely woven membranes like CO2-RSM-H (fig. S15). In stark contrast, blank membranes demonstrated a considerable shape deformation and force drop during adhesion force measurements, lacking any apparent tunable adhesion behavior and exhibiting rapid spreading of oil droplets upon the surface. LF-NMR shed light on the wettability switching mechanism, displaying an increase in confined and nanoconfined water in CO2-RSM membranes upon CO2 bubbling (fig. S16), attributed to the conversion of PDEAEMA segments into a protonated, hydrophilic, chain-extended state (19, 38). N2 purging reverted these segments to a deprotonated, hydrophobic, and chain-coiled state (39), thereby diminishing the water-capturing ability as indicated by a decrease in confined and nanoconfined water. SERS mapping further validated the CO2 response mechanism (figs. S17 and S18), revealing a potent signal from the H-bonds between H2O and protonated amine groups in PMMA-co-PDEAEMA45 upon CO2 bubbling, which disappeared post CO2 removal. The gas-tunable adhesion behavior of the CO2-RSM membrane is predominantly ascribed to its switchable surface wettability during alternate CO2/N2 stimulation.
Gas-switchable nanoemulsion separation
The excellent gas-tunable surface wettability, small average pore size, and narrow pore size distribution of CO2-RSM-H membrane make it suitable for a wide variety of applications in the field of nanoemulsion separation. Using a cross-flow filtration setup (as illustrated in fig. S19) with the effective area of 3.14 cm2, and the driving force of the separation process was 0.15 bar, and feed flow rate was 0.8 liter/min, we assessed the separation efficacy of CO2-RSM-H against a range of surfactant-stabilized nanoemulsions. These included both O/W and W/O varieties, originating from light oils like n-hexane and isooctane, as well as high-viscosity oils such as silicone oil, soybean oil, olive oil, and crude oil. As shown in Fig. 3A, the CO2-RSM-H membrane showed excellent separation performance toward both n-hexane–based O/W and W/O nanoemulsions with different droplet sizes (optical photos and raw particle size distribution data can be seen in figs. S20 to S22). As engineered, the core-shell design of the fibers equips CO2-RSM-H with a gas-responsive nanoemulsion separation capability. When stimulated by CO2, the membrane effectively separates W/O nanoemulsions, whereas N2 stimulation activates its ability to separate O/W nanoemulsions. For comparison, the blank membrane achieves a poor performance and shows no switching behavior (figs. S23 to S26).
Fig. 3. Gas-switchable nanoemulsion separation.
(A) Separation efficiency of CO2-RSM-H membrane for both W/O and O/W nanoemulsions with varying droplet sizes. The nanoemulsions were prepared using water and n-hexane, and CO2/N2 stimulation was used to switch from separating one emulsion type to the other. (B) Separation efficiency of CO2-RSM-H membrane for W/O and O/W nanoemulsions with a droplet size of 50 nm and varying viscosity. CO2/N2 stimulation was used to switch from separating one emulsion type to the other. (C) DLS analysis showing isooctane-based O/W and W/O nanoemulsion feed solutions with a droplet size of ~20 nm and the clean filtrate solutions that are produced when the feed solutions are filtered through a CO2-RSM-H membrane under CO2/N2 stimulation. The accompanying solution images display a Tyndall effect in the feed, which vanishes after filtering through the CO2-RSM-H membrane. (D) Separation efficiency of CO2-RSM-H membrane for isooctane-based nanoemulsions with the droplet size of ~20 nm under CO2/N2 stimulation. (E) Comparison of the nanoemulsion separation performance of the CO2-RSM-H membrane with state-of-the-art membranes reported in the literature. All membranes achieve >99% separation efficiency (table S1). Blue and red symbols indicate superwetting membranes specific for treating O/W or O/W nanoemulsion, respectively, whereas bicolor symbols represent stimuli-responsive superwetting membranes effective for treating both O/W and O/W nanoemulsion. Refer to table S1 for details on experimental conditions and performance data. Performance evaluations are based on the nanoemulsion sizes measured in each study. The size represents what has been tested but may not represent the performance limit of respective membranes.
CO2-RSM-H membrane’s outstanding switchable separation capability is not just confined to purifying hexane-based nanoemulsions. It can effectively handle a broad spectrum of nanoemulsions made with oils of varying viscosities (as shown in Fig. 3B and figs. S27 to S31), achieving an ultralow oil content of below 3 parts per million (ppm) for O/W nanoemulsions and water content of less than 6 ppm for W/O nanoemulsions. In addition, regardless of the water-removing or oil-removing mode, the separation efficiencies of all nanoemulsions are above 99.6%, and no obvious changes are observed over 10 cycles (fig. S32). This showcases the excellent reusability of the CO2-RSM-H membrane, which is capable of targeting virtually any nanoemulsion separation. Even for isooctane-based nanoemulsions with droplet sizes as small as 20 nm (Fig. 3, C and D), the CO2-RSM-H membrane kept satisfactory separation performance. The initial feed nanoemulsions displayed a clear Tyndall effect, but after filtration through the membrane, all the filtrates turned transparent, as illustrated in Fig. 3C. A moisture meter and total organic carbon analysis verified that the concentration of nanoemulsions in the filtrates was as low as 3 ppm (O/W) and 4 ppm (O/W), respectively, resulting in ultrahigh efficiencies above 99.6% (Fig. 3D). We compared the separation efficiency of the CO2-RSM-H membrane with those reported in the literature (Fig. 3E and table S2). The overwhelming majority of reported membranes only show high separation efficiency for nanoemulsions with droplet sizes above 100 nm. Moreover, most of them have a single separation function (i.e., either for O/W nanoemulsion or for W/O nanoemulsion purification). In contrast, our CO2-RSM-H membrane not only can effectively separate both O/W and W/O nanoemulsions but also exhibited superior separation performance for nanoemulsions with droplet sizes below 25 nm.
Separation mechanism of CO2-RSM membrane
To unravel the gas-switchable nanoemulsion separation mechanism of CO2-RSM-H, we conducted a series of molecular dynamics (MD) simulations. In a representative MD simulation depicted in Fig. 4 (A and B), specific quantities of water and isooctane molecules (30 water/1495 isooctane molecules to simulate W/O and 12,980 water/30 isooctane molecules to simulate O/W) were placed on the feed side of a ~2-nm-thick membrane to analyze their permeation. In the W/O system, the membrane’s hydrophobic surface causes water molecules to cluster (see fig. S33), as validated by the diminishing water clusters and their increasing size (Fig. 4, C and D). Conversely, isooctane molecules create a layer atop the membrane, restricting water molecules. While under CO2 stimulation in the O/W system (Fig. 4B), there is a substantial presence of hydrogen bonds. The major portion consists of hydrogen bonds formed between N atoms and water molecules, with a minor fraction involving hydrogen bonds between O atoms in the carbonyl group of the ester and water (Fig. 4, E and F). This is attributed to CO2 inducing the formation of hydrogen bonds between the lone pair electrons of N atoms on free amine molecules and water. This process renders the membrane hydrophilic, forming a water layer on the membrane surface that hinders the permeation of isooctane (fig. S34). Simultaneously, the activation of water molecules enhances their reactivity with CO2, leading to the generation of unstable HCO3− ions (fig. S35). In contrast, under N2 stimulation in the W/O system, only a small number of hydrogen bonds form between O atoms and water (fig. S36).
Fig. 4. MD simulations.
(A and B) The final conformation snapshots of a W/O emulsion with the CO2-RSM-H membrane in its hydrophobic state (A) and a O/W emulsion with the CO2-RSM-H membrane converted to its hydrophilic state using CO2 stimulation (B). Water molecules are represented using a van der Waals model (oxygen and hydrogen atoms are colored red and white, respectively) in the O/W emulsion and a red line model in the W/O emulsion. Octane molecules are represented using a blue line model in the O/W emulsion and a van der Waals model in the W/O emulsion. (C) Number of clusters and maximum cluster size for water molecules in the W/O system. (D) Change of cluster size as a function of simulation time. (E) Number of hydrogen bonds between water molecules and CO2-RSM-H in the O/W system. (F) Snapshot of the hydrogen bonds between a water molecule and the membrane. (G and H) Density of water (red line) and isooctane (blue line) molecules along the z axis of the system (perpendicular to the surface of membrane) in the W/O (G) and the O/W system (H). (I) Diffusion coefficient for water and isooctane molecules in W/O and O/W systems.
This distinct transport of the water and isooctane molecules through the membrane was further confirmed by comparing the density distribution of water and isooctane molecules in the z direction. As shown in Fig. 4G, in the W/O system, it was clearly observed that the majority of isooctane molecules exist at both sides of the membrane, while the water molecules are mainly distributed at the feed side. However, in the O/W system, an opposing trend was observed (Fig. 4H). The diffusion coefficient (D) (Fig. 4I) revealed that in the initial state, the diffusion coefficient in the z direction for water in the W/O system (Fig. 4I) is only 4.5 × 10−7 cm2/s, notably lower than isooctane diffusion (6.1 × 10−5 cm2/s). This suggests that the CO2-RSM membrane favors isooctane but hinders water transport in the W/O system. After CO2 stimulation, the diffusion coefficient for water molecules in the O/W system increased to 5.1 × 10−5 cm2/s, far exceeding isooctane diffusion (4.7 × 10−7 cm2/s). This indicates that the CO2-RSM membrane favors water but impedes isooctane transport in the O/W system. These findings align with our experimental nanoemulsion separation results (Fig. 3, A to C).
Scale-up potential of CO2-RSM membrane
To showcase the scalability of the CO2-RSM membrane, we fabricated a sizable CO2-RSM-H membrane with dimensions of 40 × 1200 cm2 (Fig. 5A). This large membrane size allowed us to assemble commercial spiral wound (SW) membrane elements such as the SW module no. 1812, a favored choice in residential water purification due to its high productivity per unit volume (40–42). The step-by-step assembly procedure is shown in Fig. 5B and figs. S37 and S38. We used this membrane element to further evaluate the continuous separation performance of the CO2-RSM-H membrane in industrially relevant conditions for different nanoemulsion systems, the effective filtration area of the membrane element is 0.2 m2, and the pressure was kept constant at 0.5 MPa, feed flow rate was 3.0 liter/min, as displayed in Fig. 5C and fig. S39. Three tanks, each containing a different type of nanoemulsion, were connected to the SW membrane element. Using strategically placed valves, the separation process could be tailored based on the emulsion type. For tank 1, which contained an n-hexane–based W/O nanoemulsion, valve 1 was opened to leverage the membrane’s inherent superhydrophobicity. After completing the separation from tank 1, valve 1 was closed, and valve 2 opened. The membrane was then bubbled with CO2 for 10 min, making it superhydrophilic, enabling the separation of the isooctane-based O/W nanoemulsion in tank 2. Last, for tank 3, valve 3 was opened, and N2 was bubbled for 20 min, preparing the membrane to separate the silicone oil–based W/O nanoemulsion. The separation efficiency of each process stabilized at >99.7%, and the contents of emulsion in the filtrates were below 6 ppm for all cases (Fig. 5D). Similar results were also achieved for other nanoemulsion systems, including isooctane-, silicone oil–, soybean oil–, hexane-, and olive oil–based O/W and W/O nanoemulsions (figs. S40 to S43). Therefore, by altering the CO2/N2 stimulation, the membrane element can achieve continuous and smart separation of various nanoemulsion mixtures. The CO2-RSM-H membrane was exposed to CO2 for only 10 min to initiate its separation characteristics change. Postactivation, it effectively operated for an additional 50 min without further CO2 exposure. Conversely, it can revert to a hydrophobic state after brief N2 exposure (fig. S44), only 200 ml of CO2 is consumed in the entire process.
Fig. 5. Scale-up of CO2-responsive superwetting membranes.
(A) Photograph of a spiral-wound membrane module showcasing a large-sized CO2-RSM-H membrane, measuring 40 × 120 cm, fully extended (top) and assembled (bottom). (B) Schematic of the spiral-wound module. (C) Schematic showing continuous and switchable separation performance of the CO2-RSM-H membrane module for different nanoemulsion systems. (D) Separation efficiency of CO2-RSM-H membrane module in different switch states. The nanoemulsions for switch 1, switch 2, and switch 3 were n-hexane–based W/O, isooctane-based O/W, and silicone oil–based W/O, respectively. The droplet size of all nanoemulsions was 50 nm. (E) Long-term stability test of the CO2-RSM-H membrane module under CO2 stimulation for the separation of crude oil–based O/W nanoemulsion with a droplet size of 50 nm. Insets show the good anti–crude oil adhesion performance of the CO2-RSM-H membrane under CO2 stimulation.
To evaluate the resilience and self-cleaning properties of the CO2-RSM-H membrane, we conducted a rigorous 30-hour stability test using crude oil–based O/W nanoemulsion, notorious for its high viscosity and tendency to cause notable membrane fouling (43, 44). Impressively, the membrane maintained consistent performance over this period; both flux and separation efficiency exhibited minimal deviations when stimulated by CO2 (Fig. 5E), underlining the membrane’s sustained nanoemulsion separation capabilities. Moreover, if fouled, the membrane can be easily cleaned thanks to its self-cleaning capabilities that are triggered by gas stimulation, leading to a change in surface wettability. As depicted in Fig. 5E and movies S1 and S2, the CO2-RSM membrane in its initial state is seriously fouled by crude oil and cannot be cleaned by water due to the strong adhesive force between crude oil and membrane surface. However, following CO2 exposure, the crude oil effortlessly detaches from the membrane’s surface, attributed to its enhanced superhydrophilic properties.
DISCUSSION
In conclusion, we present a CO2-responsive superwetting membrane by weaving CO2-responsive core-shell fibers for the effective separation of various nanoemulsions with industrially relevant droplet sizes and viscosities. The core-shell structure of fibers, combined with the weaving process, imparts the membranes the unique ability to regulate their surface wettability and pore structure under CO2/N2 stimulation. Consequently, these membranes showcase outstanding nanoemulsion separation capabilities, achieving over 99.6% separation efficiency across diverse oil/water nanoemulsions of varying droplet sizes. Our integrated theoretical and experimental approach shed light on the importance of surface wettability and pore structure in realizing high nanoemulsion separation performance. Furthermore, our methodology is conducive to large-scale production. We successfully fabricated an industrial-scale membrane with an effective area of 4800 cm2 and demonstrated that it retains high separation efficiency and excellent durability and stability. This work not only demonstrates the potential of CO2-responsive membranes in emerging water purification applications but also sets a benchmark for innovative designs and versatile adaptations of various stimuli-responsive membranes.
MATERIALS AND METHODS
Materials
Polyester fiber was provided by B. Zhu (School of Textiles Science and Clothing, Jiangnan University). The SW membrane element (no. 1812) was provided by Y. Lu (Guochu Technology (Xiamen Co. Ltd). Tetrahydrofuran, n-hexane, and isooctane were obtained from Beijing Chemical Co. Ltd. (chemical purity). Soybean oil and olive oil were purchased from Macklin Biochemical Co. Ltd. (Shanghai, Chian). Silicone oil was purchased from Zhonglan Chenguang Chemical Research and Design Institute Co. Ltd. (Sichuan, China). Crude oil was provided by Nanjing Refinery Co. Ltd. (Nanjing, China). Span80 and Tween 80 were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Nitrogen (99.99%) and carbon dioxide (99.99%) were supplied by Wuxi Xinnan Chemical Gas Co. Ltd. (China). The water used in all experiments was deionized water. All chemicals were used as received without further purification.
Methods
Preparation of CO2-responsive core-shell fibers
The typical fabrication process of CO2-responsive core-shell fibers through a one-step coating process is as follows: 7 wt % of PMMA-co-PDEAEMA copolymer, which was fabricated by our previous method (22), was first dissolved in ethanol (copolymer composition in table S1). Next, the blank polyester fibers were dried under infrared heating at 80°C and immersed into the above solution by using a sizing machine (Y606S, Nantong Sansi Mechanical and Electrical Technology Co. Ltd.). The movement speed of the fibers was 50 m min−1. Last, the resulting polyester fibers were dried in a vacuum oven at 50°C for 24 hours and automatically collected on the drum. Such coating process was conducted one to four times to prepare the CO2-responsive core-shell fibers with different shell thicknesses.
Preparation of CO2-responsive superwetting membrane
The weaving process is illustrated in Fig. 2A. We selected the CO2-responsive core-shell fiber, featuring four coatings, for constructing all CO2-responsive superwetting membranes. This weaving was conducted using a Y300S-SA rapier loom from Nantong Sansi Mechanical and Electrical Technology Co. Ltd. We used three weaving densities: 200 T (low density), 250 T (medium density), and 300 T (high density).
Acknowledgments
Funding: This study was supported by the National Key Research and Development Program of China (2021YFB3802600) (L.D.), National Natural Science Foundation of China (22278178) (L.D.), Fundamental Research Funds for the Central Universities (JUSRP622035) (L.D.), and Natural Science Foundation of Xinjiang Uygur Autonomous Region (2022D01D030) (H.M.)
Author contributions: Conceptualization: Y.W., L.D., L.F.V., and B.Z. Supervision: L.L. and L.D.. Membrane fabrication: Y.W.. Characterization and performance tests: Y.W.. Characterizations support: J.L., B.Z., and L.D. Result analysis: Y.B., C.Z., Q.-F.A., L.F.V., H.M., C.C., and L.D.. Simulation: L.L.. Writing—original draft: Y.W., L.L., and B.Z. Writing—review and editing: L.F.V., M.E., Q.-F.A., H.M., Y.Z., and L.D..
Competing interests: The authors declare that they have no competing interests.
Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.
Supplementary Materials
This PDF file includes:
Supplementary Text
Figs. S1 to S44
Tables S1 and S2
Legends for movies S1 and S2
References
Other Supplementary Material for this manuscript includes the following:
Movies S1 and S2
REFERENCES AND NOTES
- 1.Sheth T., Seshadri S., Prileszky T., Helgeson M. E., Multiple nanoemulsions. Nat. Rev. Mater. 5, 214–228 (2020). [Google Scholar]
- 2.Cherukupally P., Sun W., Wong A. P. Y., Williams D. R., Ozin G. A., Bilton A. M., Park C. B., Surface-engineered sponges for recovery of crude oil microdroplets from wastewater. Nat. Sustain. 3, 136–143 (2020). [Google Scholar]
- 3.Schrope M., The lost legacy of the last great oil spill. Nature 466, 304–305 (2010). [DOI] [PubMed] [Google Scholar]
- 4.Schrope M., Oil spill: Deep wounds. Nature 472, 152–154 (2011). [DOI] [PubMed] [Google Scholar]
- 5.Peydayesh M., Mezzenga R., Protein nanofibrils for next generation sustainable water purification. Nat. Commun. 12, 3248 (2021). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Werber J. R., Osuji C. O., Elimelech M., Materials for next-generation desalination and water purification membranes. Nat. Rev. Mater. 1, 16018 (2016). [Google Scholar]
- 7.Zhang J., Han B., Supercritical or compressed CO2 as a stimulus for tuning surfactant aggregations. Acc. Chem. Res. 2, 425–433 (2013). [DOI] [PubMed] [Google Scholar]
- 8.Anis S. F., Lalia B. S., Hashaikeh R., Hilal N., Hierarchical underwater oleophobic electro-ceramic/carbon nanostructure membranes for highly efficient oil-in-water separation. Sep. Purif. Technol. 275, 119241 (2021). [Google Scholar]
- 9.Silva H. D., Cerqueira M. A., Vicente A. A., Nanoemulsions for food applications: Development and characterization. Food Bioproc. Tech. 5, 854–867 (2012). [Google Scholar]
- 10.Hu L., Gao S., Ding X., Wang D., Jiang J., Jin J., Jiang L., Photothermal-responsive single-walled carbon nanotube-based ultrathin membranes for on/off switchable separation of oil-in-water nanoemulsions. ACS Nano 9, 4835–4842 (2015). [DOI] [PubMed] [Google Scholar]
- 11.Giorno L., Oil-in-water nanoemulsions for better nanofiltration membranes. Nat. Water 1, 228–229 (2023). [Google Scholar]
- 12.Ainsworth E. A., Rogers A., The response of photosynthesis and stomatal conductance to rising CO2: Mechanisms and environmental interactions. Plant Cell Environ. 30, 258–270 (2007). [DOI] [PubMed] [Google Scholar]
- 13.Beltran-Castillo S., Olivares M. J., Contreras R. A., Zuniga G., Llona I., von Bernhardi R., Eugenin J. L., D-serine released by astrocytes in brainstem regulates breathing response to CO2 levels. Nat. Commun. 8, 838 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Rodrigues W. P., Martins M. Q., Fortunato A. S., Rodrigues A. P., Semedo J. N., Simoes-Costa M. C., Pais I. P., Leitao A. E., Colwell F., Goulao L., Maguas C., Maia R., Partelli F. L., Campostrini E., Scotti-Campos P., Ribeiro-Barros A. I., Lidon F. C., DaMatta F. M., Ramalho J. C., Long-term elevated air CO2 strengthens photosynthetic functioning and mitigates the impact of supra-optimal temperatures in tropical Coffea arabica and C. canephora species. Glob. Chang. Biol. 22, 415–431 (2016). [DOI] [PubMed] [Google Scholar]
- 15.Xu Z., Jiang Y., Jia B., Zhou G., Elevated-CO2 response of stomata and its dependence on environmental factors. Front. Plant Sci. 7, 657 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Li X., Kang S., Niu J., Huo Z., Liu J., Improving the representation of stomatal responses to CO2 within the Penman-Monteith model to better estimate evapotranspiration responses to climate change. J. Hydrol. 572, 692–705 (2019). [Google Scholar]
- 17.Dong L., Zhao Y., CO2-switchable membranes: Structures, functions, and separation applications in aqueous medium. J. Mater. Chem. A 8, 16738–16746 (2020). [Google Scholar]
- 18.Jiang B., Zhang Y., Huang X., Kang T., Severtson S. J., Wang W.-J., Liu P., Tailoring CO2-responsive polymers and nanohybrids for green chemistry and processes. Ind. Eng. Chem. Res. 58, 15088–15108 (2019). [Google Scholar]
- 19.Cunningham M. F., Jessop P. G., Carbon dioxide-switchable polymers: Where are the future opportunities? Macromolecules 52, 6801–6816 (2019). [Google Scholar]
- 20.Che H., Huo M., Peng L., Fang T., Liu N., Feng L., Wei Y., Yuan J., CO2-responsive nanofibrous membranes with switchable oil/water wettability. Angew. Chem. Int. Ed. 54, 8934–8938 (2015). [DOI] [PubMed] [Google Scholar]
- 21.Lei L., Zhang Q., Shi S., Zhu S., Highly porous poly(high internal phase emulsion) membranes with “open-cell” structure and CO2-switchable wettability used for controlled oil/water separation. Langmuir 33, 11936–11944 (2017). [DOI] [PubMed] [Google Scholar]
- 22.Wang Y., Yang S., Zhang J., Chen Z., Zhu B., Li J., Liang S., Bai Y., Xu J., Rao D., Dong L., Zhang C., Yang X., Scalable and switchable CO2-responsive membranes with high wettability for separation of various oil/water systems. Nat. Commun. 14, 1108 (2023). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23.Li Y., Zhu L., Grishkewich N., Tam K. C., Yuan J., Mao Z., Sui X., CO2-responsive cellulose nanofibers aerogels for switchable oil- water separation. ACS Appl. Mater. Interfaces 11, 9367–9373 (2019). [DOI] [PubMed] [Google Scholar]
- 24.Abraham S., Kumaran S. K., Montemagno C. D., Gas-switchable carbon nanotube/polymer hybrid membrane for separation of oil-in-water emulsions. RSC Adv. 7, 39465–39470 (2017). [Google Scholar]
- 25.Lin S., Shang J., Theato P., Facile fabrication of CO2-responsive nanofibers from photo-cross-linked poly(pentafluorophenyl acrylate) nanofibers. ACS Macro Lett. 7, 431–436 (2018). [DOI] [PubMed] [Google Scholar]
- 26.Huang X., Mutlu H., Theato P., A CO2-gated anodic aluminum oxide based nanocomposite membrane for de-emulsification. Nanoscale 12, 21316–21324 (2020). [DOI] [PubMed] [Google Scholar]
- 27.Zhang Q., Wang Z., Lei L., Tang J., Wang J., Zhu S., CO2-switchable membranes prepared by immobilization of CO2-breathing microgels. ACS Appl. Mater. Interfaces 9, 44146–44151 (2017). [DOI] [PubMed] [Google Scholar]
- 28.Shirin-Abadi A. R., Gorji M., Rezaee S., Jessop P. G., Cunningham M. F., CO2-switchable-hydrophilicity membrane (CO2-SHM) triggered by electric potential: Faster switching time along with efficient oil/water separation. Chem. Commun. 54, 8478–8481 (2018). [DOI] [PubMed] [Google Scholar]
- 29.Zhang J., Liu Y., Guo J., Yu Y., Li Y., Zhang X., A CO2-responsive PAN/PAN-co-PDEAEMA membrane capable of cleaning protein foulant without the aid of chemical agents. React. Funct. Polym. 149, 104503 (2020). [Google Scholar]
- 30.Ma X., Li Y., Hussain I., Shen R., Yang G., Zhang K., Core-shell structured nanoenergetic materials: Preparation and fundamental properties. Adv. Mater. 32, e2001291 (2020). [DOI] [PubMed] [Google Scholar]
- 31.El-Toni A. M., Habila M. A., Labis J. P., Alothman Z. A., Alhoshan M., Elzatahry A. A., Zhang F., Design, synthesis and applications of core-shell, hollow core, and nanorattle multifunctional nanostructures. Nanoscale 8, 2510–2531 (2016). [DOI] [PubMed] [Google Scholar]
- 32.Qi J., Lai X., Wang J., Tang H., Ren H., Yang Y., Jin Q., Zhang L., Yu R., Ma G., Su Z., Zhao H., Wang D., Multi-shelled hollow micro−/nanostructures. Chem. Soc. Rev. 44, 6749–6773 (2015). [DOI] [PubMed] [Google Scholar]
- 33.Li J., Li S., Huang J., Khan A. Q., An B., Zhou X., Liu Z., Zhu M., Spider silk-inspired artificial fibers. Adv. Sci. 9, e2103965 (2022). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 34.Keten S., Xu Z., Ihle B., Buehler M. J., Nanoconfinement controls stiffness, strength and mechanical toughness of beta-sheet crystals in silk. Nat. Mater. 9, 359–367 (2010). [DOI] [PubMed] [Google Scholar]
- 35.Li Z., Zhu Y.-L., Niu W., Yang X., Jiang Z., Lu Z.-Y., Liu X., Sun J., Healable and recyclable elastomers with record-high mechanical robustness, unprecedented crack tolerance, and superhigh elastic restorability. Adv. Mater. 33, e2101498 (2021). [DOI] [PubMed] [Google Scholar]
- 36.Ling S., Kaplan D. L., Buehler M. J., Nanofibrils in nature and materials engineering. Nat. Rev. Mater. 3, 18016 (2018). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 37.Koh L.-D., Cheng Y., Teng C.-P., Khin Y.-W., Loh X.-J., Tee S.-Y., Low M., Ye E., Yu H.-D., Zhang Y.-W., Han M.-Y., Structures, mechanical properties and applications of silk fibroin materials. Prog. Polym. Sci. 46, 86–110 (2015). [Google Scholar]
- 38.Yan Q., Zhao Y., Block copolymer self-assembly controlled by the “green” gas stimulus of carbon dioxide. Chem. Commun. 50, 11631–11641 (2014). [DOI] [PubMed] [Google Scholar]
- 39.Kozlovskaya E. N., Pitsevich G. A., Malevich A. E., Doroshenko O. P., Pogorelov V. E., Doroshenko I. Y., Balevicius V., Sablinskas V., Kamnev A. A., Raman spectroscopic and theoretical study of liquid and solid water within the spectral region 1600-2300 cm−1. Spectrochim. Acta A 196, 406–412 (2018). [DOI] [PubMed] [Google Scholar]
- 40.Chen X., Liu G., Jin W., Natural gas purification by asymmetric membranes: An overview. Green Energy Environ. 6, 176–192 (2021). [Google Scholar]
- 41.Koros W. J., Zhang C., Materials for next-generation molecularly selective synthetic membranes. Nat. Mater. 16, 289–297 (2017). [DOI] [PubMed] [Google Scholar]
- 42.Mauter M. S., Zucker I., Perreault F., Werber J. R., Kim J.-H., Elimelech M., The role of nanotechnology in tackling global water challenges. Nat. Sustain. 1, 166–175 (2018). [Google Scholar]
- 43.Zhang S., Jiang G., Gao S., Jin H., Zhu Y., Zhang F., Jin J., Cupric phosphate nanosheets-wrapped inorganic membranes with superhydrophilic and outstanding anticrude oil-fouling property for oil/water separation. ACS Nano 12, 795–803 (2018). [DOI] [PubMed] [Google Scholar]
- 44.Peng B., Yao Z., Wang X., Crombeen M., Sweeney D. G., Tam K. C., Cellulose-based materials in wastewater treatment of petroleum industry. Green Energy Environ. 5, 37–49 (2020). [Google Scholar]
- 45.Spoel D. V. D., Lindahl E., Hess B., Groenhof G., Mark A. E., Berendsen H. J. C., GROMACS: Fast, flexible, and free. J. Comput. Chem. 26, 1701–1718 (2005). [DOI] [PubMed] [Google Scholar]
- 46.Darden T., York D., Pedersen L., Particle mesh Ewald: AnN⋅log(N) method for Ewald sums in large systems. J. Chem. Phys. 98, 10089–10092 (1993). [Google Scholar]
- 47.Berendsen H. J. C., Postma J. P. M., Van Gunsteren W. F., DiNola A., Haak J. R., Molecular dynamics with coupling to an external bath. J. Chem. Phys. 81, 3684–3690 (1984). [Google Scholar]
- 48.Humphrey W., Dalke A., Schulten K., VMD: Visual molecular dynamics. J. Mol. Graph. 14, 33–38 (1996). [DOI] [PubMed] [Google Scholar]
- 49.Wang X., Sun K., Zhang G., Yang F., Lin S., Dong Y., Robust zirconia ceramic membrane with exceptional performance for purifying nano-emulsion oily wastewater. Water Res. 208, 117859 (2022). [DOI] [PubMed] [Google Scholar]
- 50.Anis S. F., Lalia B. S., Lesimple A., Hashaikeh R., Hilal N., Superhydrophilic and underwater superoleophobic nano zeolite membranes for efficient oil-in-water nanoemulsion separation. J. Water Process Eng. 40, 101802 (2021). [Google Scholar]
- 51.Gao S. J., Zhu Y. Z., Zhang F., Jin J., Superwetting polymer-decorated SWCNT composite ultrathin films for ultrafast separation of oil-in-water nanoemulsions. J. Mater. Chem. A 3, 2895–2902 (2015). [Google Scholar]
- 52.Zhang L., Lin Y., Cheng L., Yang Z., Matsuyama H., A comprehensively fouling- and solvent-resistant aliphatic polyketone membrane for high-flux filtration of difficult oil-in-water micro- and nanoemulsions. J. Membr. Sci. 582, 48–58 (2019). [Google Scholar]
- 53.Zhan H., Zuo T., Tao R., Chang C., Robust tunicate cellulose nanocrystal/palygorskite nanorod membranes for multifunctional oil/water emulsion separation. ACS Sustainable Chem. Eng. 6, 10833–10840 (2018). [Google Scholar]
- 54.Pi J.-K., Yang J., Xu Z.-K., One-pot mussel-inspiration and silication: A platform for constructing oil-repellent surfaces toward crude oil/water separation. J. Membr. Sci. 601, 117915 (2020). [Google Scholar]
- 55.Wang Z., Ji S., Zhang J., He F., Xu Z., Peng S., Li Y., Dual functional membrane with multiple hierarchical structures (MHS) for simultaneous and high-efficiency removal of dye and nano-sized oil droplets in water under high flux. J. Membr. Sci. 564, 317–327 (2018). [Google Scholar]
- 56.Xi J., Lou Y., Chu Y., Meng L., Wei H., Dai H., Xu Z., Xiao H., Wu W., High-flux bacterial cellulose ultrafiltration membrane with controllable pore structure. Colloids Surf. A Physicochem. Eng. Asp. 656, 130428 (2023). [Google Scholar]
- 57.Qin D., Liu Z., Bai H., Sun D., Song X., A new nano-engineered hierarchical membrane for concurrent removal of surfactant and oil from oil-in-water nanoemulsion. Sci. Rep. 6, 24365 (2016). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 58.Ashrafi Z., Lucia L., Krause W., Underwater superoleophobic matrix-formatted liquid-infused porous biomembranes for extremely efficient deconstitution of nanoemulsions. ACS Appl. Mater. Interfaces 12, 50996–51006 (2020). [DOI] [PubMed] [Google Scholar]
- 59.Peng K., Wang C., Chang C., Peng N., Phosphonium modified nanocellulose membranes with high permeate flux and antibacterial property for oily wastewater separation. Coatings 12, 1598 (2022). [Google Scholar]
- 60.Li D., Huang X., Huang Y., Yuan J., Huang D., Cheng G. J., Zhang L., Chang C., Additive printed all-cellulose membranes with hierarchical structure for highly efficient separation of oil/water nanoemulsions. ACS Appl. Mater. Interfaces 11, 44375–44382 (2019). [DOI] [PubMed] [Google Scholar]
- 61.Ye X., Ke L., Wang Y., Gao K., Cui Y., Wang X., Huang X., Shi B., Polyphenolic-chemistry-enabled, mechanically robust, flame resistant and superhydrophobic membrane for separation of mixed surfactant-stabilized emulsions. Chemistry 24, 10953–10958 (2018). [DOI] [PubMed] [Google Scholar]
- 62.Du J., Zhou C., Chen L., Cheng J., Pi P., Zuo J., Shen W., Jin S., Tan L., Dong L., Gate-embedding strategy for pore size manipulation on stainless steel mesh: Toward highly efficient water-in-oil nanoemulsions separation. Ind. Eng. Chem. Res. 58, 15288–15296 (2019). [Google Scholar]
- 63.Chen L., Si Y., Zhu H., Jiang T., Guo Z., A study on the fabrication of porous PVDF membranes by in-situ elimination and their applications in separating oil/water mixtures and nano-emulsions. J. Membr. Sci. 520, 760–768 (2016). [Google Scholar]
- 64.Hu L., Gao S., Zhu Y., Zhang F., Jiang L., Jin J., An ultrathin bilayer membrane with asymmetric wettability for pressure responsive oil/water emulsion separation. J. Mater. Chem. A 3, 23477–23482 (2015). [Google Scholar]
- 65.Cai Y., Chen D., Li N., Xu Q., Li H., He J., Lu J., A smart membrane with antifouling capability and switchable oil wettability for high-efficiency oil/water emulsions separation. J. Membr. Sci. 555, 69–77 (2018). [Google Scholar]
- 66.Cheng L., Wang D. M., Shaikh A. R., Fang L. F., Jeon S., Saeki D., Zhang L., Liu C. J., Matsuyama H., Dual superlyophobic aliphatic polyketone membranes for highly efficient emulsified oil-water separation: Performance and mechanism. ACS Appl. Mater. Interfaces 10, 30860–30870 (2018). [DOI] [PubMed] [Google Scholar]
- 67.Long Y., Shen Y., Tian H., Yang Y., Feng H., Li J., Superwettable coprinus comatus coated membranes used toward the controllable separation of emulsified oil/water mixtures. J. Membr. Sci. 565, 85–94 (2018). [Google Scholar]
- 68.Du L., Quan X., Fan X., Chen S., Yu H., Electro-responsive carbon membranes with reversible superhydrophobicity/superhydrophilicity switch for efficient oil/water separation. Sep. Purif. Technol. 210, 891–899 (2019). [Google Scholar]
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Tables S1 and S2
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References
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