Aqueous-organic Interface Research Articles

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.

Effect of ion to ligand ratio on the aqueous to organic relative solubility of a lanthanide-ligand complex.

In the solvent extraction of rare earth elements, mechanistic aspects remain unclear regarding where and how extractant molecules coordinate metal ions and transport them from the aqueous phase into the organic phase. Molecular dynamics simulations were used to examine how unprotonated di(2-ethylhexyl)phosphoric acid (DEHP-) ligands that coordinate the Gd3+ ion can transfer the ion across the water-organic interface. Using the umbrella sampling technique, potential of mean force profiles were constructed to quantify the relative solubility of the Gd3+ ion coordinated to 0-3 DEHP- ligands in either water, 1-octanol, or hexane solvents and at the water-organic interfaces. The simulations show the Gd-DEHP- complexes, at varying Ln-ligand ratios, preferentially solvate on water-organic interfaces. While the Gd(DEHP-)3 complex will diffuse past the aqueous-organic interface into the octanol solvent, it is thermodynamically preferred for the Gd(DEHP-)3 complex to remain in the water-hexane interface when there is no amphiphilic layer of excess ligand.

Investigation of interfacial mass transfer phenomena applying non-invasive Raman imaging and density gradient theory

This study presents a novel non-invasive Raman measurement setup for investigating diffusion and mass transfer processes at liquid/liquid interfaces, based on a photometric setup utilizing customized photon multiplier (CPMs). The results highlight the effectiveness, reliability and applicability of this setup in visualizing and analyzing acetone mass transfer across the aqueous-organic interface. The study focuses on the liquid/liquid extraction of acetone from the aqueous phase into the organic phase, using toluene as the organic solvent. The study's key findings indicate that – in the absence of forced convection – mass transfer across the interface is not the limiting factor in the extraction process; instead, diffusion in the bulk phase takes precedence. Acetone's rapid local transport from the aqueous to the organic phase is observed, with a temporary concentration minimum on the aqueous side of the interface. In the organic phase, local equilibrium is reached near the interface first, only after which acetone disperses into the bulk phase. This dual mechanism—fast interfacial mass transfer and slower bulk-phase diffusion—marks a significant feature of the considered extraction process, which were further substantiated by dynamic concentration gradient theory simulations. The research further demonstrates the feasibility of upscaling interfacial mass transfer simulations using adaptive meshing. However, macroscopic effects like induced convection challenge purely diffusive models. The simulations confirm rapid enrichment of acetone at the interface, followed by slower mass transfer through the interface and bulk phase diffusion, consistent with the experimental data. Experimental and modeled results align well with literature data for liquid/liquid extraction, thereby validating the accuracy of the presented novel diffusion measurement setup and demonstrating its potential to investigate mass transfer phenomena near liquid/liquid interfaces.

Electromigration separation of lithium isotopes: The effect of ionic liquid ratios

This study is based on the system of “anodic aqueous solution | crown ether-ionic liquid organic solution | cathodic aqueous solution”, the influence of ionic liquid ratio in organic solution under different voltages on Li+ transport and lithium isotope separation was investigated. It is found that ionic liquids have multiple roles. In addition to improving the conductivity of organic solutions and enhancing Li+ transport capacity between aqueous solution and organic solution through ion exchange. Ionic liquids are even to realize the migration of lithium ions from aqueous solution to organic solution through solvation. There is an obvious synergistic enhancement between ionic liquid solvation and electric field. Based on this, three types of Li+ transport across the aqueous-organic interface are proposed. The influencing mechanism of ionic liquid ratio, voltage on Li+ migration ratio and lithium isotope separation is discussed in detail.

Boosting the performance of reverse osmosis membrane by macrocyclic molecule through interfacial coordination-driven strategy

Developing polyamide reverse osmosis (RO) membrane with superior permeability and salt rejection is highly desired for desalination. In this study, we propose an interfacial coordination-driven strategy to promote the incorporation of macrocyclic molecule into the polyamide layer and further improve the performance of RO membrane. To this end, the macrocyclic molecule Noria and transition metal salt Fe(acac)3 are added into the aqueous phase and organic phase respectively. During the interfacial polymerization (IP) reaction of m-phenylenediamine (MPD) with trimesoyl chloride (TMC), Fe3+ as electron acceptor would coordinate with the electron donor Noria to form Noria-Fe3+ complex at the aqueous-organic interface. This effectively promotes the diffusion of Noria and its further incorporation into the resulting polyamide layer. As a result, the RO membrane with Noria-Fe3+ exhibits closely packed band-like surface morphology, obviously different from the traditional leaf-like structure formed by MPD-TMC. Benefitting from the inner cavity of Noria, the permeance of RO membrane is significantly improved from 1.37 Lm−2h−1bar−1 to 3.87 Lm−2h−1 bar−1 at the optimal Noria and Fe(acac)3 concentrations of 0.1 wt%, while the NaCl rejection experiences little change from 98.8 % to 98.2 %. Besides, the flux decline ratio and flux recovery ratio of obtained RO membrane are 21 % and 85 % respectively in the bovine serum albumin fouling test. It exhibits more excellent anti-fouling ability than the nascent membrane due to the better hydrophilicity and smoother membrane surface. This simple strategy may pave a novel way for the application of other molecules with special structures as additives to fabricate high-performance RO membranes.

Highly Luminescent and Stable Halide Perovskite Nanocrystals by Interfacial Defect Passivation and Amphiphilic Ligand Capping.

Halide vacancies cause lattice degradation and nonradiative losses in halide perovskites. In this study, we strategically fill bromide vacancies in CsPbBr3 perovskite nanocrystals with NaBr, KBr, or CsBr at the organic-aqueous interface for hydrophobic ligand-capped nanocrystals or in a polar solvent (2-propanol) for amphiphilic ligand-capped nanocrystals. Energy-dispersive X-ray spectra, powder X-ray diffraction data, and scanning transmission electron microscopy images help us confirm vacancy filling and the structures of samples. The bromide salts increase the photoluminescence quantum yield (98 ± 2%) of CsPbBr3 by decreasing the nonradiative decay rate. Single-particle studies show the quantum yield increase originates from the poorly luminescent nanocrystals becoming highly luminescent after filling vacancies. Furthermore, we tune the optical band gap (ultraviolet-visible-near-infrared) of the hydrophobic ligand-capped nanocrystals by halide exchange at the toluene-water interface using saturated NaCl or NaI solutions, which completes in about 60 min under continuous mixing. In contrast, the amphiphilic ligand accelerates the halide exchange in 2-propanol, suggesting ambipolar functional groups speed up the ion-exchange reaction. The bromide vacancy-filled or halide-exchanged samples in a toluene-water biphasic solvent show higher stability than amphiphilic ligand-capped samples in 2-propanol. This strategy of defect passivation, ion exchange, and ligand chemistry to improve quantum yields and tune band gaps of halide perovskite nanocrystals can be promising for designing stable and water-soluble perovskite samples for solar cells, light-emitting diodes, photodetectors, and photocatalysts.

Kinetics of ultrasound-assisted solvent extraction of vanadium with methyltrioctylammonium chloride (MAC)

The ultrasound-assisted extraction process of vanadium from the aqueous phase to the organic phase by methyltrioctylammonium chloride (MAC) was studied. The factors that influence the vanadium extraction rate such as stirring speed, temperature, and ultrasound power were investigated. The rate expression of vanadium extraction with ultrasound was established. The extraction process could consist of diffusion and chemical capturing steps at the aqueous-organic interface. This study demonstrated that the diffusion resistance could be eliminated by increasing the stirring speed and ultrasound power. The vanadium extraction process without diffusion resistance conformed to the first-order kinetic model. The activation energy (Ea) of this extraction process was determined as 36.7 kJ mol−1 along with a pre-exponential factor k0a as 3.27 × 105 s−1, while the ultrasound enhancement factor can be expressed as ε = 1 + 1.40 × 10−2P - 6.61 × 10−5P2. The optimal extraction kinetics are identified by examining the factors affecting the vanadium extraction rate (rextr), which are stirring speed ≥ 200 rpm, ultrasound power (P) = 90 W, extraction temperature (T) = 318.15 K. This study provides a theoretical basis for the extraction process of vanadium and aids the design of ultrasound-assisted extraction applications.

Rapid synthesis of ultrathin covalent organic polymer membranes with subnanometer pores for efficient organic solvent nanofiltration

Covalent organic polymers (COPs) with abundant micropores are deemed as promising materials for building molecular separation membranes. In spite of this potential, fabricating COP separation membranes through an efficient and scalable method remains a significant challenge. Herein, we report a simple and efficient strategy for the synthesis of COP membranes with three-dimensionally interconnected micropores. Utilizing transition-metal nitrates as a catalyst, we achieve fast polymerization of amorphous COPs with relatively uniform micropores (0.7 nm in diameter). The rational design of organic-aqueous interface allows for the direct fabrication of robust COP membranes with an ultrathin thickness of ∼20 nm on porous polyacrylonitrile substrates. The resulting hydrophobic but pore-uniform frameworks in these membranes permit fast permeation of organic liquids with a notable molecular weight cutoff of 388 g mol−1 in ethanol. To demonstrate scalability, we show that our strategy can produce large-size COP membranes with a prominent area of 200 cm2, which exhibit performances similar to that of small membrane coupons. The present study offers a potentially scalable method for producing highly microporous COP membranes toward efficient molecular separation in organic liquids.

Radiolytic evaluation of acetohydroxamic acid (AHA) under biphasic (HNO3:n-dodecane and TBP/DEHBA/DEHiBA) used nuclear fuel reprocessing conditions

Acetohydroxamic acid (AHA) has been proposed as a substitute for hydrazine-stabilized reductants for the selective separation of plutonium and neptunium from co-extracted uranium during the reprocessing of used nuclear fuel. However, limited information exists for the chemical behavior of AHA in ionizing radiation fields, especially under representative biphasic reprocessing solvent system conditions. Here we present a systematic gamma irradiation study on the radiolytic integrity of AHA in aqueous nitric acid solutions (0.2 M HNO3) in contact with an organic phase, comprising current (tri-butyl phosphate) and next generation (N,N-di-(2-ethylhexyl)butyramide and N,N-di-(2-ethylhexyl)isobutyramide) reprocessing ligands dissolved in n-dodecane diluent. Biphasic AHA irradiations exhibited negligible effect of the organic phase on the rate of AHA radiolysis—affording an average pseudo dose constant of d' = (−58.66 ± 6.10) × 10−4 kGy−1—and subsequent formation of the predominant degradation products (hydroxylamine and acetic acid), relative to complementary single-phase systems. The radiolysis of AHA was found to have negligible impact on ligand degradation rates: G(TBP) = −0.038 ± 0.002 μmol J−1, G(DEHBA) = −0.117 ± 0.004 μmol J−1, and G(DEHiBA) = −0.102 ± 0.003 μmol J−1. These observations demonstrate that the radiolytic transients and steady-state degradation products generated in either phase have limited capacity for migrating across the aqueous-organic interface to undergo subsequent chemistry within the investigated dose range (≤110 kGy). These findings support the substitution of AHA into advanced reprocessing flowsheets.

One-Step Construction of the Positively/Negatively Charged Ultrathin Janus Nanofiltration Membrane for the Separation of Li+ and Mg2+

To coordinate the trade-off between the separation and permeation of the nanofiltration membrane for the separation of Mg2+/Li+, we regulated poly(ethyleneimine)/piperazine interface polymerization parameters to construct a positively/negatively charged ultrathin Janus nanofiltration membrane at a free aqueous-organic interface. At the optimized interfacial polymerization parameters, 0.03 wt % of piperazine reacted with trimethylbenzene chloride prior to poly(ethyleneimine), forming a primary polyamide layer with fewer defects or limiting large-scale defects of the polyamide layer. The controlled subsequent reaction of poly(ethyleneimine) and trimethylbenzene chloride results in a Janus nanofiltration membrane, with one side enriched with the carboxyl groups, the other side enriched with the amine groups, and a dense polyamide structure in the middle. Under the optimum conditions, the positive potential of the rear surface of the prepared membrane was 14.57 mV, and the water contact angle reached 71.31°, while the negative potential of the front surface was -25.48 mV, and the water contact angle was 12.93°, confirming a Janus membrane with opposite charges and large hydrophilicity differences in the front and rear surfaces. With a high cross-linking degree, a 40 nm thick polyamide layer is 29.09% more thinner than the traditional polyamide membrane. The ultrathin Janus nanofiltration membrane showed an excellent separation factor (SLi,Mg of 18.26), stability, and water permeability flux (10.6 L·m-2·h-1·bar-1). The rejections to MgCl2, CaCl2, MgSO4, and Na2SO4 are measured above 90% at a nearly constant permeability of 10.6 L·m-2·h-1·bar-1, particularly stable rejections to MgCl2 and Na2SO4.

Polyamide nanofilms synthesized by a sequential process of blade coating-spraying-interfacial polymerization toward reverse osmosis

Thin film composite (TFC) membranes composed of polyamide nanofilms and porous substrates have received enhancing attention to desalination for overcoming the nexus of serious scarcity and huge demand for fresh water. Up to now, it is still a big challenge to avoid the complex interference of the porous substrates on the interfacial polymerization of amine and acyl chloride monomers for synthesizing polyamide nanofilms at lower carbon footprint. Herein, we report a sequential process of blade coating–spraying–interfacial polymerization for scalable preparation of polyamide nanofilms that shine in high-performance desalination. Our sequential process consists of 1) blade coating the viscous solution of water/glycerol/m-phenylenediamine (MPD) to form an anti-interference aqueous film, 2) spaying the organic solution of Isopar H/trimesoyl chloride (TMC) under a controlled and economical way onto the water/glycerol/MPD film, and 3) conducting the interfacial polymerization of MPD and TMC at the constructed organic-aqueous interface. The whole process highly reduces the utilization of organic solvents to meet the demands of lower carbon footprint. The synthesized polyamide nanofilms exhibit extremely low roughness and 10 nm thickness in an area larger than 200 cm2, which can be composited with various porous substrates. The as-prepared TFC membranes represent a high desalination performance of water permeance of 1.89 L m−2 h−1 bar−1 with 97.4% NaCl rejection. Our strategy broadens the way to large-scale synthesis of polyamide nanofilms for high-efficiency desalination membranes.

Cycloaddition of CO2 to epoxides “around water”: a strategy to apply and recycle efficient water-soluble bio-based organocatalysts in biphasic media

A biphasic reaction approach is presented for the cycloaddition of CO2 to epoxides where the catalyst is dissolved in water droplets and the reaction occurs at the aqueous–organic interface allowing easy recycling of the catalyst by decantation.

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.

Demystifying viscous isoalkanes as the organic solvent in interfacial polymerization for manufacturing desalination membranes

It is deemed as a silver bullet that the mass production of desalination membranes will solve the exacerbated water scarcity, which normally uses polyamide nanofilm as the perm-selective layer through interfacial polymerization. Although there have been significant breakthroughs in laboratory research in recent years, the same good results cannot all be reproduced owing to many differences between pilot tests and mass production, such as the use of organic solvents. Herein, a new perspective is proposed for elucidating the deep reason that viscous isoalkanes are welcomed in the industrial production of desalination membranes. Interfacial polymerization of amine and acyl chloride is dissected from the initial stage including the distribution and enrichment of monomers at the aqueous–organic interface, as well as the diffusion and solvation state of polyamide oligomers. The viscous isoalkanes not only suppress the diffusion of acyl chloride monomer but also provide a better solvation environment for polyamide oligomers. Polyamide nanofilms with reduced thickness and roughness are obtained with an enhanced cross-linking density. The corresponding desalination membranes exhibit improved water permeance (~3 Lm−2h−1bar−1) and rejection to NaCl (~99 %) that surpasses the performance of free-standing polyamide nanofilms recently reported. Our facile strategy will facilitate the transformation of laboratory results into the mass production of customized high-performance desalination membranes.

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 Na2SO4 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.

Separation of Pd(II) and Pt(IV) in concentrated hydrochloric acid using thin-layer oil membrane and comparison with conventional extraction

This study proposes a new thin-layer oil membrane (TOM) extraction strategy for enhanced mutual separation of Pd(II) and Pt(IV) chlorocomplex anions in concentrated aqueous hydrochloric acid solutions using di-n-butyl sulfoxide (DBSO) as an extractant. It was discovered that Pd(II) could be preferentially extracted even in concentrated hydrochloric acid solutions. The separation coefficient between Pd(II) and Pt(IV) considerably increased than that between them during conventional extraction using a separating funnel. A new extractor was designed to perform TOM extraction by spreading a thin layer of organic extractant liquid membrane on the surface of aqueous feed solutions under laminar flow. A simplified TOM extraction model and controlled experiments were used to investigate the effect of H+ and Cl− ion concentrations on the mutual separation between Pd(II) and Pt(IV). Molecular dynamics (MD) simulations were used to understand the microscopic separation mechanisms at the molecular level. Experimental results revealed that the competitive distribution of H+ and Cl− ions in the boundary layer along the normal vertical direction of the organic-aqueous interface are crucial in enhancing the separation between Pd(II) and Pt(IV). The competitive adsorption and enrichment of Cl− ions at the interface prevent DBSO molecules from being protonated by H+ ions, which is beneficial for the substituted coordination of DBSO molecules with PdCl42−. However, the ionic association of protonated DBSO molecules with PtCl62− remains dominant at the interface. As a result, even in concentrated hydrochloric acid solutions, the difference in mass transfer between PdCl42− and PtCl62− toward the interface was intensified.

Permeation Process of Metal Complexes with Hydroxyoxime and Carboxylic Acid Extractants through Organic-Aqueous Interface

Permeation Process of Metal Complexes with Hydroxyoxime and Carboxylic Acid Extractants through Organic-Aqueous Interface

Fast calcium carbonate film growth induced by 1-naphthoic acid at the organic-aqueous phase

Inducing calcium carbonate (CaCO3) precipitation is an active research field for its functional extension. Herein, we presented a simple but efficient method for fast preparation of CaCO3 thin film induced by 1-naphthoic acid (1 N) at organic-aqueous interface. The film structure was characterized by scanning electron microscopy (SEM) and X-ray powder diffractometry. Grain growth and morphology of the quickly formed thin film are described. The results indicate the thickness of the film can reach 5–20 µm, with promotion and acceleration of mineralization between liquid–liquid interface induced by 1 N. These results shed light on the importance of organic molecule-induced crystallization of CaCO3 and open a novel synthetic route for its precipitation.

Mix-charged polyamide membranes via molecular hybridization for selective ionic nanofiltration

Nanofiltration membranes that can selectively remove multivalent ions from salty water hold great promise in water treatment and salt utilization. However, developing nanofiltration membranes with high rejection to both divalent anions and cations remains a grand challenge. Herein, we design a kind of mix-charged polyamide membranes (MCPMs) via molecular hybridization to achieve the removal of versatile divalent ions. Amino-functionalized polyhedral oligomeric silsesquioxane (NH2-POSS) was employed as molecular filler added into piperazine and reacted with trimesoyl chloride at the aqueous-organic interface to generate cross-linked polyamide-NH2-POSS mixed network. The protonated amino groups from NH2-POSS offered positively charged sites for enhanced divalent cation repulsion while the deprotonated carboxyl groups from polyamide network can afford negatively charged sites for divalent anion repulsion. The optimized membrane displayed high rejections to both divalent anionic and cationic ions (Na2SO4 rejection of 98.2%, MgCl2 rejection of 98.0%), together with high mono-/divalent salt selectivity (NaCl/Na2SO4 of 48.2, LiCl/MgCl2 of 43.9) that outperformed the benchmark nanofiltration membranes. This molecularly-hybridized charge modulation method may enlighten the structural design of membranes for efficient ionic separations.

Modulating the pro-apoptotic activity of cytochrome c at a biomimetic electrified interface.

Programmed cell death via apoptosis is a natural defence against excessive cell division, crucial for fetal development to maintenance of homeostasis and elimination of precancerous and senescent cells. Here, we demonstrate an electrified liquid biointerface that replicates the molecular machinery of the inner mitochondrial membrane at the onset of apoptosis. By mimicking in vivo cytochrome c (Cyt c) interactions with cell membranes, our platform allows us to modulate the conformational plasticity of the protein by simply varying the electrochemical environment at an aqueous-organic interface. We observe interfacial electron transfer between an organic electron donor decamethylferrocene and O2, electrocatalyzed by Cyt c. This interfacial reaction requires partial Cyt c unfolding, mimicking Cyt c in vivo peroxidase activity. As proof of concept, we use our electrified liquid biointerface to identify drug molecules, such as bifonazole, that can potentially down-regulate Cyt c and protect against uncontrolled neuronal cell death in neurodegenerative disorders.