Ultrathin Film Composite Membranes Fabricated by Novel In Situ Free Interfacial Polymerization for Desalination

Distinct Efficacies of Interlayers in Tailoring Polyamide Nanofiltration Membrane Performance for Organic Micropollutant Removal: Dependent on Substrate Characteristics

Interlayered thin-film nanocomposite (TFN) membranes have shown the potential to boost nanofiltration performance for water treatment applications including the removal of organic micropollutants (OMPs). However, the effects of substrates have been overlooked when exploiting and evaluating the efficacy of certain kinds of interlayers in tailoring membrane performance. Herein, a series of TFN membranes were synthesized on different porous substrates with identical interlayers of metal-organic framework nanosheets. It was revealed that the interlayer introduction could narrow but not fully eliminate the difference in the properties among the polyamide layers formed on different substrates, and the membrane performance variation was prominent in distinct aspects. For substrates with small pore sizes exerting severe water transport hindrance, the introduced interlayer mainly enhanced membrane water permeance by affording the gutter effect, while it could be more effective in reducing membrane pore size by improving the interfacial polymerization platform and avoiding PA defects when using a large-pore-size substrate. By matching the selected substrates and interlayers well, superior TFN membranes were obtained with simultaneously higher water permeance and OMP rejections compared to three commercial membranes. This study helps us to objectively understand interlayer efficacies and attain performance breakthroughs of TFN membranes for more efficient water treatment.

Antifouling Asymmetric Block Copolymer Nanofilms via Freestanding Interfacial Polymerization for Efficient and Sustainable Water Purification

Membrane materials that resist nonspecific or specific adsorption are urgently required in widespread practical applications, such as water purification, food processing, and life sciences. In water purification, inevitable membrane fouling not only limits membrane separation performance, leading to a decline in both permeance and selectivity, but also remarkably increases operation requirements, and augments extra maintenance costs and higher energy consumption. In this work, we report a freestanding interfacial polymerization (IP) fabrication strategy for in situ creation of asymmetric block copolymer (BCP) nanofilms with antifouling properties, greatly outperforming the conventional surface post-modification approaches. The resultant free-standing asymmetric BCP nanofilms with highly-dense, highly-hydrophilic polyethylene glycol (PEG) brushes on one side, can be readily formed via a typical IP process of a well-defined double-hydrophilic BCP composed of a highly-efficient antifouling PEG block and a membrane-forming multiamine block. The asymmetric BCP nanofilms have been applied for efficient and sustainable natural water purification, demonstrating extraordinary antifouling capabilities accompanied with superior separation performance far beyond commercial polyamide nanofiltration membranes. The antifouling behaviors of asymmetric BCP nanofilms derived from the combined effect of the hydration layer, electrostatic repulsion and steric hindrance were further elucidated by water flux and fouling resistance in combination with all-atom molecular dynamics (MD) simulation. This work opens up a new avenue for the large-scale and low-cost creation of broad-spectrum, asymmetric membrane materials with diverse functional "defect-free" surfaces in real-world applications.

Empowering ultrathin polyamide membranes at the water-energy nexus: strategies, limitations, and future perspectives

Membrane-based separation is one of the most energy-efficient methods to meet the growing need for a significant amount of fresh water. It is also well-known for its applications in water treatment, desalination, solvent recycling, and environmental remediation. Most typical membranes used for separation-based applications are thin-film composite membranes created using polymers, featuring a top selective layer generated by employing the interfacial polymerization technique at an aqueous-organic interface. In the last decade, various manufacturing techniques have been developed in order to create high-specification membranes. Among them, the creation of ultrathin polyamide membranes has shown enormous potential for achieving a significant increase in the water permeation rate, translating into major energy savings in various applications. However, this great potential of ultrathin membranes is greatly hindered by undesired transport phenomena such as the geometry-induced "funnel effect" arising from the substrate membrane, severely limiting the actual permeation rate. As a result, the separation capability of ultrathin membranes is still not fully unleashed or understood, and a critical assessment of their limitations and potential solutions for future studies is still lacking. Here, we provide a summary of the latest developments in the design of ultrathin polyamide membranes, which have been achieved by controlling the interfacial polymerization process and utilizing a number of novel manufacturing processes for ionic and molecular separations. Next, an overview of the in-depth assessment of their limitations resulting from the substrate membrane, along with potential solutions and future perspectives will be covered in this review.

Leveraging Janus Substrates as a Confined "Interfacial Reactor" to Synthesize Ultrapermeable Polyamide Nanofilms

Porous substrates act as open "interfacial reactors" during the synthesis of polyamide composite membranes via interfacial polymerization. However, achieving a thin and dense polyamide nanofilm with high permeance and selectivity is challenging when using a conventional substrate with uniform wettability. To overcome this limitation, we propose the use of Janus porous substrates as confined interfacial reactors to decouple the local monomer concentration from the total monomer amount during interfacial polymerization. By manipulating the location of the hydrophilic/hydrophobic interface in a Janus porous substrate, we can precisely control the monomer solution confined within the hydrophilic layer without compromising its concentration. The hydrophilic surface ensures the uniform distribution of monomers, preventing the formation of defects. By employing Janus substrates fabricated through single-sided deposition of polydopamine/polyethyleneimine, we significantly reduce the thickness of the polyamide nanofilms from 88.4 to 3.8 nm by decreasing the thickness of the hydrophilic layer. This reduction leads to a remarkable enhancement in water permeance from 7.2 to 52.0 l/m2·h·bar while still maintaining ~96% Na2SO4 rejection. The overall performance of this membrane surpasses that of most reported membranes, including state-of-the-art commercial products. The presented strategy is both simple and effective, bringing ultrapermeable polyamide nanofilms one step closer to practical separation applications.

Bendable and Chemically Stable Metal-Organic Hybrid Membranes for Molecular Separation

Crystalline porous metal-organic materials are ideal building blocks for separation membranes because of their molecular-sized pores and highly ordered pore structure. However, creating ultrathin, defect-free crystalline membranes is challenging due to inevitable grain boundaries. Herein, we reported an amorphous metal-organic hybrid (MOH) membrane with controlled microporosity. The synthesis of the MOH membrane entails the use of titanium alkoxide and organic linkers containing di/multicarboxyl groups as monomers in the polymerization reaction. The resultant membranes exhibit similar microporosity to existing molecular sieve materials and high chemical stability against harsh chemical environments owing to the formation of stable Ti-O bonds between metal centers and organic linkers. An interfacial polymerization is developed to fabricate an ultrathin MOH membrane (thickness of the membrane down to 80 nm), which exhibits excellent rejections (>98% for dyes with molecular weights larger than 690 Da) and high water permeance (55 L m-2 h-1 bar-1). The membranes also demonstrate good flexibility, which greatly improves the processability of the membrane materials.

Harmonic amide bond density as a game-changer for deciphering the crosslinking puzzle of polyamide

It is particularly essential to analyze the complex crosslinked networks within polyamide membranes and their correlation with separation efficiency for the insightful tailoring of desalination membranes. However, using the degree of network crosslinking as a descriptor yields abnormal analytical outcomes and limited correlation with desalination performance due to imperfections in segmentation and calculation methods. Herein, we introduce a more rational parameter, denoted as harmonic amide bond density (HABD), to unravel the relationship between the crosslinked networks of polyamide membranes and their desalination performance. HABD quantifies the number of distinct amide bonds per unit mass of polyamide, based on a comprehensive segmentation of polyamide structure and consistent computational protocols derived from X-ray photoelectron spectroscopy data. Compared to its counterpart, HABD overcomes the limitations and offers a more accurate depiction of the crosslinked networks. Empirical data validate that HABD exhibits the expected correlation with the salt rejection and water permeance of reverse osmosis and nanofiltration polyamide membranes. Notably, HABD is applicable for analyzing complex crosslinked polyamide networks formed by highly functional monomers. By offering a powerful toolbox for systematic analysis of crosslinked polyamide networks, HABD facilitates the development of permselective membranes with enhanced performance in desalination applications.

Polyamide nanofiltration membranes by vacuum-assisted interfacial polymerization: Broad universality of Substrate, wide window of monomer concentration and high reproducibility of performance

Vacuum assistance is used for filtering solid substances onto porous substrates to create composite membranes typically. However, the potential of this approach has rarely been assessed in facilitating the distribution of liquids within those porous substrates to fabricate composite membranes in typical interfacial polymerization. In this work, we demonstrate the advantages of vacuum-assisted interfacial polymerization (VAIP) in terms of substrate universality, monomer concentration range, and performance reproducibility in the fabrication of polyimide nanofiltration membranes. Aqueous solutions of PIP can be homogeneously distributed by vacuum filtration on diverse microfiltration substrates of polyether sulfone (PES), Nylon-66, polyvinylidene fluoride (PVDF), cellulose acetate (CA), and mixed cellulose esters (MCE), respectively. Interfacial polymerization is then performed on these substrates using different concentrations of piperazine (PIP, 0.0075-0.1000 wt%) and trimoyl chloride (TMC, 0.0112-0.1500 wt%). Remarkably, a uniform and ultra-thin polyamide layer with a thickness of 15 nm can be achieved at an exceptionally low PIP concentration of 0.0250 wt%, exhibits a rejection rate of over 98.8 % for Na2SO4 and a water permeance of 25.8 L·m-2·h-1·bar-1. The membranes with a diameter of 30 cm demonstrate reproducibility in nanofiltration performance and satisfactory long-term stability. This method offers a simple yet effective strategy for regulating the liquid distribution and optimizing interfacial polymerization in fabricating polyamide composite membranes.

Interfacial polymerization at unconventional interfaces: an emerging strategy to tailor thin-film composite membranes

Interfacial polymerization is a well-known process to synthesize separation layers for thin film composite membranes at an immiscible organic liquid-aqueous liquid interface. The organic-aqueous interface determines the diffusion dynamics of monomers and the chemical environment for polymerization, exerting a critical influence on the formation of polymer thin films. This review summarizes recent advances in tailoring interfacial polymerization using interfaces beyond the conventional alkane-water interface to achieve high-performance separation films with designed structures. Diverse liquid-liquid interfaces are introduced for synthesizing separation films by adding co-solvents into the organic phase and/or the aqueous phase, respectively, or by replacing one of the liquid phases with other solvents. Innovative liquid-gel and liquid-gas interfaces are then summarized for the synthesis of polymer thin films for separation. Novel strategies to form reaction interfaces, such as spray-coating, are also presented and discussed. In addition, we discuss the details of how a physically or chemically patterned substrate affects interfacial polymerization. Finally, the potential of unconventional interfaces in interfacial polymerization is forecast with both challenges and opportunities.

Thin-Film Composite Membrane with Porous Interlayer Composed of Dendritic Mesoporous Silica Nanoparticles for Enhanced Nanofiltration

Positively charged nanofiltration (NF) membranes show great potential in the fields of water treatment and resource recovery. However, this kind of NF membrane usually suffers from relatively low water permeance. Herein, a positively charged NF membrane with a porous interlayer is developed, where the interlayer is formed by assembling dendritic mesoporous silica nanoparticles (DMSNs) after the formation of a polyamide layer. This post-assembly strategy avoids the adverse effect of the interlayer on the formation of positively charged NF membranes. The porous DMSN interlayer provides abundant connected channels for water transport, thus endowing the NF membrane with enhanced water permeance. A series of DMSNs with different sizes was synthesized, and their influence on membrane formation and membrane performance was systematically investigated. The optimized membrane exhibits a CaCl2 rejection rate of 95.2% and a water flux of 133.6 L·h-1·m-2, which is 1.6 times that of the control group without an interlayer. This work represents an approach to the fabrication of a positively charged NF membrane with porous interlayers for high-efficiency cation rejection.

Quantitatively Unveiling the Structure-Activity Relationship of Polyamide Nanofiltration Membranes with Complex Structures

Nanofiltration polyamide (NF PA) membranes are widely used in seawater desalination and wastewater treatment due to their excellent permeability. The structure-activity relationship of PA membranes has attracted extensive attention in decades. In this work, NF PA membranes with planar structure, nodular structure, and peak-valley structure were constructed, and the pure water permeance was calculated by nonequilibrium molecular dynamics simulation to quantitatively investigate the structure-activity relationship between the microstructure and water permeance. Results showed that the peak-valley structure had the highest effective utilization rate of the membrane surface, which had the highest number of water molecules that passed through membranes per unit cross-sectional area (7.09). Furthermore, with the increase of the specific surface area ratio, the water permeance of the NF PA with peak-valley increased at a rate about 2.5 times than that of the planar NF PA. Therefore, some molecular scale insights were supplied about the structure-activity relationship of NF PA membranes, which is helpful for the fabrication of high-performance NF PA membranes.

NaHCO3 addition enhances water permeance and Ca/haloacetic acids selectivity of nanofiltration membranes for drinking water treatment

The existence of disinfection by-products such as haloacetic acids (HAAs) in drinking water severely threatens water safety and public health. Nanofiltration (NF) is a promising strategy to remove HAAs for clean water production. However, NF often possesses overhigh rejection of essential minerals such as calcium. Herein, we developed highly selective NF membranes with tailored surface charge and pore size for efficient rejection of HAAs and high passage of minerals. The NF membranes were fabricated through interfacial polymerization (IP) with NaHCO3 as an additive. The NaHCO3-tailored NF membranes exhibited high water permeance up to ∼24.0 L m - 2 h - 1 bar-1 (more than doubled compared with the control membrane) thanks to the formation of stripe-like features and enlarged pore size. Meanwhile, the tailored membranes showed enhanced negative charge, which benefitted their rejection of HAAs and passage of Ca and Mg. The higher rejection of HAAs (e.g., > 90%) with the lower rejection of minerals (e.g., < 30% for Ca) allowed the NF membranes to achieve higher minerals/HAAs selectivity, which was significantly higher than those of commercially available NF membranes. The simultaneously enhanced membrane performance and higher minerals/HAAs selectivity would greatly boost water production efficiency and water quality. Our findings provide a novel insight to tailor the minerals/micropollutants selectivity of NF membranes for highly selective separation in membrane-based water treatment.

Control the hydrophilic layer thickness of Janus membranes by manipulating membrane wetting in membrane distillation

Janus membranes with asymmetric wettability have attracted wide attentions for their robust anti-oil-wetting/fouling abilities in membrane distillation (MD). Compared to traditional surface modification approaches, in this study, we provided a new approach which manipulated surfactant-induced wetting to fabricate Janus membrane with a controllable thickness of the hydrophilic layer. The membranes with 10, 20, and 40 μm of wetted layers were obtained by stopping the wetting induced by 40 mg L^-1 Triton X-100 (J = 25 L m^-2 h^-1) at about 15, 40, and 120 s, respectively. Then, the wetted layers were coated using polydopamine (PDA) to fabricate the Janus membranes. The resulting Janus membranes showed no significant change in porosities or pore size distributions compared with the virgin PVDF membrane. These Janus membranes exhibited low in-air water contact angles (< 50°), high underwater oil contact angles (> 145°), and low adhesion with oil droplets. Therefore, they all showed excellent oil-water separation performance with ∼100% rejection and stable flux. The Janus membranes showed no significant decline in flux, but a trade-off existed between the hydrophilic layer thicknesses and the vapor flux. Utilizing membranes with tunable hydrophilic layer thickness, we elucidated the underlying mechanism of such trade-off in mass transfer. Furthermore, the successful modification of membranes with different coatings and in-situ immobilization of silver nanoparticles indicated that this facile modification method is universal and can be further expanded for multifunctional membrane fabrication.

When self-assembly meets interfacial polymerization

Interfacial polymerization (IP) and self-assembly are two thermodynamically different processes involving an interface in their systems. When the two systems are incorporated, the interface will exhibit extraordinary characteristics and generate structural and morphological transformation. In this work, an ultrapermeable polyamide (PA) reverse osmosis (RO) membrane with crumpled surface morphology and enlarged free volume was fabricated via IP reaction with the introduction of self-assembled surfactant micellar system. The mechanisms of the formation of crumpled nanostructures were elucidated via multiscale simulations. The electrostatic interactions among m-phenylenediamine (MPD) molecules, surfactant monolayer and micelles, lead to disruption of the monolayer at the interface, which in turn shapes the initial pattern formation of the PA layer. The interfacial instability brought about by these molecular interactions promotes the formation of crumpled PA layer with larger effective surface area, facilitating the enhanced water transport. This work provides valuable insights into the mechanisms of the IP process and is fundamental for exploring high-performance desalination membranes.

High-Performance Polyamide Reverse Osmosis Membrane Containing Flexible Aliphatic Ring for Water Purification

A reverse osmosis (RO) membrane with a high water permeance and salt rejection is needed to reduce the energy requirement for desalination and water treatment. However, improving water permeance while maintaining a high rejection of the polyamide RO membrane remains a great challenge. Herein, we report a rigid-flexible coupling strategy to prepare a high-performance RO membrane through introducing monoamine with a flexible aliphatic ring (i.e., piperidine (PPR)) into the interfacial polymerization (IP) system of trimesoyl chloride (TMC) and m-phenylenediamine (MPD). The resulted polyamide film consists of a robust aromatic skeleton and soft aliphatic-ring side chain, where the aliphatic ring optimizes the microstructure of polyamide network at a molecular level. The obtained membranes thereby showed an enhanced water permeance of up to 2.96 L·m^-2 h^-1 bar^-1, nearly a 3-fold enhancement compared to the control group, meanwhile exhibiting an ultrahigh rejection toward NaCl (99.4%), thus successfully overcoming the permeability-selectivity trade-off limit. Furthermore, the mechanism of the enhanced performance was investigated by molecular simulation. Our work provides a simple way to fabricate advanced RO membranes with outstanding performance.

Enhanced Separation Performance of Polyamide Thin-Film Nanocomposite Membranes with Interlayer by Constructed Two-Dimensional Nanomaterials: A Critical Review

Thin-film composite (TFC) polyamide (PA) membrane has been widely applied in nanofiltration, reverse osmosis, and forward osmosis, including a PA rejection layer by interfacial polymerization on a porous support layer. However, the separation performance of TFC membrane is constrained by the trade-off relationship between permeability and selectivity. Although thin-film nanocomposite (TFN) membrane can enhance the permeability, due to the existence of functionalized nanoparticles in the PA rejection layer, the introduction of nanoparticles leads to the problems of the poor interface compatibility and the nanoparticles agglomeration. These issues often lead to the defect of PA rejection layers and reduction in selectivity. In this review, we summarize a new class of structures of TFN membranes with functionalized interlayers (TFNi), which promises to overcome the problems associated with TFN membranes. Recently, functionalized two-dimensional (2D) nanomaterials have received more attention in the assembly materials of membranes. The reported TFNi membranes with 2D interlayers exhibit the remarkable enhancement on the permeability, due to the shorter transport path by the "gutter mechanism" of 2D interlayers. Meanwhile, the functionalized 2D interlayers can affect the diffusion of two-phase monomers during the interfacial polymerization, resulting in the defect-free and highly crosslinked PA rejection layer. Thus, the 2D interlayers enabled TFNi membranes to potentially overcome the longstanding trade-off between membrane permeability and selectivity. This paper provides a critical review on the emerging 2D nanomaterials as the functionalized interlayers of TFNi membranes. The characteristics, function, modification, and advantages of these 2D interlayers are summarized. Several perspectives are provided in terms of the critical challenges for 2D interlayers, managing the trade-off between permeability, selectivity, and cost. The future research directions of TFNi membranes with 2D interlayers are proposed.

Boosting brackish water treatment via integration of mesoporous γ-Al2O3NPs with thin-film nanofiltration membranes

In this study, a simple method based on non-ionic surfactant polysorbates-80 was used to create mesoporous γ-Al₂O₃NPs. The properties of the prepared mesoporous alumina nanoparticles (Al₂O₃NPs) were verified using ATR-FTIR, XRD, SEM, TEM, DLS, and BET surface area analysis. Then, thin-film nanocomposite (TFN) nanofiltration membranes were fabricated by interfacial polymerization of embedded polyamide layers with varied contents (0.01 to 0.15 wt.%) of mesoporous γ-Al₂O₃NPs. The surface roughness, porosity, pore size, and contact angle parameters of all the prepared membranes were also determined. The performance of the fabricated membranes was investigated under various mesoporous γ-Al₂O₃NPs loads, time, and pressure conditions. Mesoporous γ-Al₂O₃NPs revealed an important role in raising both the membrane hydrophilicity and the surface negativity. The addition of 0.03 wt.% mesoporous γ-Al₂O₃NPs to the TFN membrane increased water flux threefold compared to the TF control (TFC) membrane, with maximum water flux reaching 96.5, 98, 60, and 52 L/(m².h) for MgSO₄, MgCl₂, Na₂SO₄, and NaCl influent solutions, respectively, with the highest salt rejection of 96.5%, 92.2%, 98.4%. The TFN-Al₂O₃ membrane was also able to soften water and remove polyvalent cations such as Mg²⁺ with a highly permeable flux. The TFN-Al₂O₃ membrane successfully removed the hardness of the applied water samples below the WHO limit compared to using merely the TFC membrane. Furthermore, the TFN-Al₂O₃ nanofiltration membrane unit proved to be a promising candidate for the desalination of real brine like that collected from the Safaga area, Egypt.

Integrating graphene oxide into layers of PVDF/PVDF@cross-linked sodium alginate/polyamide membrane for efficiently enhancing desalination performances

The membrane modules of the water treatment system are faced costly damages; thereby executing pre-desalination units based on Nanofiltration (NF) could prevent these suffers, and improve the permeated water flux (PWF) and salt rejection (SR). Hence, we focused on the construction of a novel ternary-layer NF membrane through “electrospinning Polyvinylidene Fluoride (PVDF) (as bottom layer)”, “generating middle layer by electrospinning PVDF along with, the implementation cross-linking after electrospraying Sodium Alginate”, and “synthesizing Polyamide (as top layer) through interfacial polymerization”. More importantly, it anticipated that the Taguchi statistical method can expeditiously optimize the effects of Graphene Oxide nano-sheets (GOns) on water-dependent properties, such as PWF and SR. Astonishingly, the desalination capabilities significantly improved, when the top, middle, and bottom layers simultaneously had 1, 0.1, and 0.1 wt.% of GOns, respectively. Overall, comparing the performances between the optimized sample containing low-dosage and without GOns demonstrated the PWF ameliorated from 6.68 to 20.36 L/m² h; also, the SR ability remained on an incremental basis as NaCl < MgCl₂ < MgSO₄ under 6 bar pressure. Manifestly, these authentic results denoted promising, innovative, and large-scaling insights when effectual PWF and SR be necessary.

Nanofiltration Membranes with Crumpled Polyamide Films: A Critical Review on Mechanisms, Performances, and Environmental Applications

Nanofiltration (NF) membranes have been widely applied in many important environmental applications, including water softening, surface/groundwater purification, wastewater treatment, and water reuse. In recent years, a new class of piperazine (PIP)-based NF membranes featuring a crumpled polyamide layer has received considerable attention because of their great potential for achieving dramatic improvements in membrane separation performance. Since the report of novel crumpled Turing structures that exhibited an order of magnitude enhancement in water permeance ( Science 2018, 360 (6388), 518-521), the number of published research papers on this emerging topic has grown exponentially to approximately 200. In this critical review, we provide a systematic framework to classify the crumpled NF morphologies. The fundamental mechanisms and fabrication methods involved in the formation of these crumpled morphologies are summarized. We then discuss the transport of water and solutes in crumpled NF membranes and how these transport phenomena could simultaneously improve membrane water permeance, selectivity, and antifouling performance. The environmental applications of these emerging NF membranes are highlighted, and future research opportunities/needs are identified. The fundamental insights in this review provide critical guidance on the further development of high-performance NF membranes tailored for a wide range of environmental applications.

Aligned macrocycle pores in ultrathin films for accurate molecular sieving

Polymer membranes are widely used in separation processes including desalination¹, organic solvent nanofiltration^2,3 and crude oil fractionation^4,5. Nevertheless, direct evidence of subnanometre pores and a feasible method of manipulating their size is still challenging because of the molecular fluctuations of poorly defined voids in polymers⁶. Macrocycles with intrinsic cavities could potentially tackle this challenge. However, unfunctionalized macrocycles with indistinguishable reactivities tend towards disordered packing in films hundreds of nanometres thick^7-9, hindering cavity interconnection and formation of through-pores. Here, we synthesized selectively functionalized macrocycles with differentiated reactivities that preferentially aligned to create well-defined pores across an ultrathin nanofilm. The ordered structure was enhanced by reducing the nanofilm thickness down to several nanometres. This orientated architecture enabled direct visualization of subnanometre macrocycle pores in the nanofilm surfaces, with the size tailored to ångström precision by varying the macrocycle identity. Aligned macrocycle membranes provided twice the methanol permeance and higher selectivity compared to disordered counterparts. Used in high-value separations, exemplified here by enriching cannabidiol oil, they achieved one order of magnitude faster ethanol transport and threefold higher enrichment than commercial state-of-the-art membranes. This approach offers a feasible strategy for creating subnanometre channels in polymer membranes, and demonstrates their potential for accurate molecular separations.

Ultrafast Interfacial Self-Assembly toward Supramolecular Metal-Organic Films for Water Desalination

Supramolecular metal-organic materials are considered as the ideal candidates for membrane fabrication due to their excellent film forming characteristics, diverse metal centers and ligand sources, and designable structure and function. However, it remains challenging to rapidly construct highly permeable supramolecular metal-organic membranes with high salt rejection. Herein, a novel ultrafast interfacial self-assembly strategy to prepare supramolecular metal-organic films through the strong coordination interaction between highly active 1,3,5-triformylphloroglucinol (TFP) ligands and Fe³⁺ , Sc³⁺ , or Cu²⁺ at the organic-aqueous interface is reported. Benefiting from the self-completing and self-limiting characteristics of this interfacial self-assembly, the new kind of supramolecular membrane with optimized composition can be assembled within 3.5 min and exhibits ultrathin, dense, defect-free features, and thus shows an excellent water permeance (21.5 L m^-2  h^-1  bar^-1 ) with a high Na₂ SO₄ rejection above 95%, which outperforms almost all of the non-polyamide membranes and commercially available nanofiltration membranes. This strong-coordination interfacial self-assembly method will open up a new way for the development of functional metal-organic supramolecular films for high-performance membrane separation and beyond.

Ultrathin Membranes for Separations: A New Era Driven by Advanced Nanotechnology

Ultrathin membranes are at the forefront of membrane research, offering great opportunities in revolutionizing separations with ultrafast transport. Driven by advanced nanomaterials and manufacturing technology, tremendous progresses are made over the last 15 years in the fabrications and applications of sub-50 nm membranes. Here, an overview of state-of-the-art ultrathin membranes is first introduced, followed by a summary of the fabrication techniques with an emphasis on how to realize such extremely low thickness. Then, different types of ultrathin membranes, categorized based on their structures, that is, network, laminar, or framework structures, are discussed with a focus on the interplays among structure, fabrication methods, and separation performances. Recent research and development trends are highlighted. Meanwhile, the performances and applications of current ultrathin membranes for representative separations (gas separation and liquid separation) are thoroughly analyzed and compared. Last, the challenges in material design, structure construction, and coordination are given, in order to fully realize the potential of ultrathin membranes and facilitate the translation from scientific achievements to industrial productions.

Ultrathin Water-Cast Polymer Membranes for Hydrogen Purification

Among various H₂ purification technologies, the use of membrane technology has been considered an ecofriendly approach for addressing the increasing hydrogen demand. Although many H₂-selective membrane materials have been reported, processing them into hollow fibers or thin-film composites (TFCs) via traditional methods either affects the performance of the materials or renders their further processing into applicable membrane forms infeasible. Herein, we propose a water-casting method for fabricating TFC membranes for hydrogen purification with high permselectivity. The film integrity and thickness were manipulated by controlling the spreadability of the casting solution, and the resultant water-cast TFC membrane that comprised an ∼30 nm selective layer demonstrated high H₂ permeance and H₂/CH₄ selectivity of approximately 190 GPU and 100, respectively, under optimized conditions. We performed a mixed-gas permeation test using a simulated off-gas of steam-methane reforming from natural gas in a single-stage system and obtained hydrogen gas of >99 mol % purity. This indicates not only the suitability of the water-cast membranes for satisfying the demand for pure hydrogen as a fuel and chemical reagent but also the great potential of the water-casting method for high-performance membranes in various industrial and environmental applications.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.