Supercritical or compressed CO2 as a stimulus for tuning surfactant aggregations

Scalable weaving of resilient membranes with on-demand superwettability for high-performance nanoemulsion separations

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.

Green solvent systems for material syntheses and chemical reactions

It is of great significance to develop environmentally benign, non-volatile and recyclable green solvents for different applications. This feature article overviews the properties of green solvent systems (e.g., ionic liquids, supercritical carbon dioxide, deep eutectic solvents and mixed green solvent systems) and their applications in (1) framework material syntheses, including metal-organic frameworks, covalent organic frameworks and hydrogen-bonded organic frameworks, and (2) CO2 conversion reactions, including photocatalytic and electrocatalytic reduction reactions. Finally, the future perspective for research on green solvent systems is proposed from different aspects.

Pickering emulsions stabilized by metal-organic frameworks, graphitic carbon nitride and graphene oxide

Pickering emulsion is a heterogeneous system consisting of at least two immiscible liquids, which are stabilized by solid particles, in which organic solvent or water is dispersed into other phase in form of micrometre-sized droplets. Compared to traditional emulsions stabilized by surfactant, solids are cheap and can be easily separated and recycled by centrifugation or filtration after use. Moreover, the properties of Pickering emulsions can be adjusted by using different types of solid particles. Up to now, Pickering emulsions have been applied in a wide range of areas such as material science and catalysis. Here we review recent studies on Pickering emulsions stabilized by metal-organic framework, graphitic carbon nitride and graphene oxide.

Ultra-thin g-C3N4/MFM-300(Fe) heterojunctions for photocatalytic aerobic oxidation of benzylic carbon centers

In situ growth of the metal-organic framework material MFM-300(Fe) on an ultra-thin sheet of graphitic carbon nitride (g-C3N4) has been achieved via exfoliation of bulk carbon nitride using supercritical CO2. The resultant hybrid structure, CNNS/MFM-300(Fe), comprising carbon nitride nanosheets (CNNS) and MFM-300(Fe), shows excellent performance towards photocatalytic aerobic oxidation of benzylic C-H groups at room temperature under visible light. The catalytic activity is significantly improved compared to the parent g-C3N4, MFM-300(Fe) or physical mixtures of both. This facile strategy for preparing heterojunction photocatalysts demonstrates a green pathway for the efficient and economic oxidation of benzylic carbons to produce fine chemicals.

Bio-Compatible Ca-BDC/Polymer Monolithic Composites Templated from Bio-Active Ca-BDC Co-Stabilized CO2-in-Water High Internal Phase Emulsions

Because of the nontoxic solvents contained in CO2-in-water emulsions, porous polymer composites templated from these emulsions are conducive for bio-applications. Herein, bio-active rod-like calcium-organic framworks (Ca-BDC MOFs, BDC= 1,4-benzenedicarboxylate anion) particles co-stabilized CO2-in-water high internal phase emulsion (C/W HIPE) in the presence of polyvinyl alcohol (PVA) is first presented. After curing of the continuous phase, followed by releasing CO2, integral 3D macro-porous Ca-BDC monolith and Ca-BDC/Poly(2-hydroxyethyl methacrylate-co-acrylamide) HIPEs monolithic composites [Ca-BDC/P(AM-co-HEMA)HIPEs] with open-cell macro-porous structures were successfully prepared. The pore structure of these porous composite can be tuned by means of tailoring the Ca-BDC dosage, carbon dioxide pressure, and continuous phase volume fractions in corresponding C/W HIPEs. Results of bio-compatibility tests show that these Ca-BDC/P(AM-co-HEMA)HIPEs monoliths have non-cytotoxicity on HepG2 cells; also, the E. coli can grow either on the surfaces or inside these monoliths. Furthermore, immobilization of β-amylase on these porous composite presents that β-amylase can be well-anchored into the porous polymer composites, its catalytic activity can be maintained even after 10 cycles. This work combined bio-active MOFs Ca-BDC, bio-compatible open-cell macroporous polymer PAM-co-HEMA and green C/W HIPEs to present a novel and facile way to prepare interconnected macro-porous MOFs/polymer composites. Compared with the existing other well-known materials such as hydrogels, these porous composites possess well-defined tunable pore structures and superior bio-activity, thereby have promising applications in bio-tissue engineering, food, and pharmaceutical.

Room-Temperature Synthesis of Covalent Organic Framework (COF-LZU1) Nanobars in CO2 /Water Solvent

The development of facile, rapid, low-energy, environmentally benign routes for the synthesis of covalent organic frameworks (COFs) is of great interest. This study concerns the utilization of water containing dissolved CO₂ as a solvent for the room-temperature synthesis of COF. The as-synthesized particles, denoted COF-LZU1, combine advantages of good crystallinity, nanoscale size, and high surface area, which suggests promising application as a support for heterogeneous catalysts. Moreover, this versatile CO₂ -assisted method is also applicable for the room-temperature synthesis of Cu-COF-LZU1. This method gives rise to new opportunities for fabricating COFs and COF-based materials with different compositions and structures.

The effect of compressed CO2 on the self-assembly of surfactants for facile preparation of ordered mesoporous carbon materials

The effect of compressed CO₂ on the properties of ordered mesoporous carbon (OMC) was investigated based on the self-assembly of surfactants in aqueous solution under mild conditions, and the acidic or basic conditions commonly used in traditional methods were substituted by compressed CO₂. Compressed CO₂ acts as both a physiochemical additive and a reagent to produce an acid catalyst in the synthesis. This new one-pot assembly approach can efficiently adjust the porous characteristics of OMC by employing different amounts of compressed CO₂, and the self-assembly mechanism is proposed. The spherical micelles formed by triblock copolymer Pluronic F127 serve as a structure-directing agent for the controllable synthesis of nanomaterials. Resorcinol/phloroglucinol and formaldehyde are used as carbon-yielding components. It was found that CO₂ can penetrate into the hydrocarbon-chain region of the F127 micelles, leading to template swelling and influencing the properties of OMC. The surfactant and precursors attracted by H-bonding interactions self-assemble and produce OMC after polymerization and carbonization. The resulting OMC as a supercapacitor electrode material exhibits outstanding specific capacitances, and the electrochemical performances change as the structural properties are varied.

CO2/Water Emulsions Stabilized by Partially Reduced Graphene Oxide

Using functional materials to stabilize emulsions of carbon dioxide (CO₂) and water is a promising way to expand the utility of CO₂ and functional materials. Here we demonstrate for the first time that the partially reduced graphene oxide (rGO) can well stabilize the emulsion of CO₂ and water without the aid of any additional emulsifier or surface modification for rGO. More interestingly, such a novel kind of emulsion provides a facile and versatile route for constructing highly porous three-dimensional rGO materials, including rGO, metal/rGO, and metal oxide/rGO networks. The as-synthesized Au/rGO composite is highly active in catalyzing 4-nitrophenol reduction and styrene epoxidation.

Redox-Responsive Self-Assembly Micelles from Poly(N-acryloylmorpholine-block-2-acryloyloxyethyl ferrocenecarboxylate) Amphiphilic Block Copolymers as Drug Release Carriers

Novel well-defined redox-responsive ferrocene-containing amphiphilic block copolymers (PACMO-b-PAEFC) were synthesized by ATRP, with poly(N-acryloylmorpholine) (PACMO) as hydrophilic blocks and poly(2-acryloyloxyethyl ferrocenecarboxylate) (PAEFC) as hydrophobic blocks. The copolymers were characterized by FT-IR and ¹H NMR spectroscopies and gel permeation chromatography, and the crystalline behavior was determined by X-ray diffraction and small-angle X-ray scattering. The results showed that the size of the lamellar crystals and crystallinity vary with the systematic compositions while the periodic structure of the lamellar stacks has no obvious change. These block copolymers could self-assemble and form globular nanoscaled core-shell micellar aggregates in aqueous solution. The reductive ferrocene groups could be changed into hydrophilic ferrocenium via mild oxidation, whereas the polymer micelles at the oxidation state could reversibly recover from their original states upon reduction by vitamin C. The tunable redox response was investigated and verified by transmission electron microscopy, ultraviolet-visible spectroscopy, cyclic voltammetry, and dynamic light scattering measurements. The copolymer micelles were used to entrap anticancer drug paclitaxel (PTX), with high drug encapsulation efficiency of 61.4%, while the PTX-loaded drug formulation exhibited oxidation-controlled drug release, and the release rate could be mediated by the kinds and concentrations of oxidants. MTT assay was performed to disclose the biocompatibility and security of the copolymer micelles and to assess anticancer efficiency of the PTX-loaded nanomicelles. The developed copolymer nanomicelles with reversible redox response are anticipated to have potential in targeted drug delivery systems for cancer therapy.

Effective and Reversible Switching of Emulsions by an Acid/Base-Mediated Redox Reaction

To develop a fast, effective, and reversible strategy for phase separation and re-emulsification of the surfactant-based emulsions, a strategy for using acid/base-mediated redox reactions was established to switch the emulsions formed from a redox-responsive anionic surfactant of potassium dodecyl seleninate (C₁₂SeO₂K). Upon acidification, C₁₂SeO₂K was reduced by KI to give didodecyl diselenide (C₁₂Se)₂, a state of almost no surface or interfacial activity; upon basification, (C₁₂Se)₂ was oxidized by I₂ to give C₁₂SeO₂K again. The fractional conversion of C₁₂SeO₂K in the reversible switching processes was close to 100%. Consequently, an unusually large change in interfacial tension (ΔIFT) as high as ∼27.1 mN m^-1 was obtained at a wider concentration range starting from the critical micelle concentration of C₁₂SeO₂K; the highest IFT at the oil-water interface was obtained after an almost complete switch-off, giving an oil-aqueous solution interface very similar to that without any emulsifiers, which leads to the effective and fast phase separation of the C₁₂SeO₂K-based switchable emulsions.

Compressed CO2 mediated synthesis of bifunctional periodic mesoporous organosilicas with tunable porosity

A facile and green method is proposed for the fabrication of bifunctional periodic mesoporous organosilicas (PMOs) using compressed CO2.

CO2-responsive polymeric materials: synthesis, self-assembly, and functional applications

CO2 is an ideal trigger for switchable or stimuli-responsive materials because it is benign, inexpensive, green, abundant, and does not accumulate in the system. Many different CO2-responsive materials including polymers, latexes, solvents, solutes, gels, surfactants, and catalysts have been prepared. This review focuses on the preparation, self-assembly, and functional applications of CO2-responsive polymers. Detailed discussion is provided on the synthesis of CO2-responsive polymers, in particular using reversible deactivation radical polymerization (RDRP), formerly known as controlled/living radical polymerization (CLRP), a powerful technique for the preparation of well-defined (co)polymers with precise control over molecular weight distribution, chain-end functional groups, and polymer architectural design. Self-assembly in aqueous dispersed media is highlighted as well as emerging potential applications.

Supercritical Fluid-Driven Polymer Phase Separation for Microlens with Tunable Dimension and Curvature

Microlenses are highly sought as reliable means for high-resolution optical imaging at low illumination intensities. Plano-convex configuration with tunable dimension and curvature is an essential feature in the microlens fabrication. In this study, we present a facile and green route for preparing well-defined microlenses based on polymer phase separation in the presence of supercritical carbon dioxide (scCO2). The behaviors of linear polymethylmethacrylate protruded from cross-linked silicone network in scCO2 environment are investigated from the perspectives of thermodynamics and kinetics. Microlenses with dimensions from 2 to 15 μm and contact angles from 55° to 112° are successfully obtained through the adjustment of the kinetic conditions and outgassing rate. With the tunable focal length, they exhibit intrinsic function of discerning submicroscale patterns that are unable to be observed directly under optical microscope. Moreover, size confinement on the substrate results in the generation of well-ordered microlens arrays, affording great promise for applications in bioimaging, photolithography, light harvesting, and optical nanosensing.

Solvation of Esters and Ketones in Supercritical CO2

Vibrational Raman spectra for the C═O stretching modes of three esters with different functional groups (methyl, a single phenyl, and two phenyl groups) were measured in supercritical carbon dioxide (scCO2). The results were compared with Raman spectra for three ketones involving the same functional groups, measured at the same thermodynamic states in scCO2. The peak frequencies of the Raman spectra of these six solute molecules were analyzed by decomposition into the attractive and repulsive energy components, based on the perturbed hard-sphere theory. For all solute molecules, the attractive energy is greater than the repulsive energy. In particular, a significant difference in the attractive energies of the ester-CO2 and ketone-CO2 systems was observed when the methyl group is attached to the ester or ketone. This difference was significantly reduced in the solute systems with a single phenyl group and was completely absent in those with two phenyl groups. The optimized structures among the solutes and CO2 molecules based on quantum chemical calculations indicate that greater attractive energy is obtained for a system where the oxygen atom of the ester is solvated by CO2 molecules.

CO2-in-PEG emulsion-templating synthesis of poly(acrylamide) with controllable porosity and their use as efficient catalyst supports

Porous poly(acrylamide)s nanoparticles prepared from CO₂-in-PEG emulsions have high catalytic activity for benzene hydrogenation reaction.

Synthesis of crosslinked polymeric nanocapsules using catanionic vesicle templates stabilized by compressed CO2

The synthesis of polymeric nanocapsules in the approximate diameter range 40-100 nm (TEM/SEM) using catanionic surfactant vesicle templates stabilized by subcritical CO2 is demonstrated. Near equimolar aqueous solutions of the surfactants sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) experienced immediate vesicle destabilization and precipitation in the absence of CO2. However, pressurization with CO2 (5 MPa) dramatically enhanced the stability of the initial vesicles, and enabled swelling of the bilayers with hydrophobic monomers via diffusion loading (loading of monomers into preformed bilayers). Subsequent radical crosslinking polymerization of the monomers n-butyl methacrylate/tert-butyl methacrylate/ethylene glycol dimethacrylate contained within the bilayers was conducted at room temperature using UV-initiation under CO2 pressure. The hollow structure of the resultant nano-objects was confirmed by successful encapsulation and retention of the dye Nile Blue. It is demonstrated that using this method, polymeric nanocapsules can be successfully prepared using diffusion loading of up to 94 wt% monomer (rel. to surfactant) stabilized by CO2.

High-efficiency exfoliation of layered materials into 2D nanosheets in switchable CO2/Surfactant/H2O system

Layered materials present attractive and important properties due to their two-dimensional (2D) structure, allowing potential applications including electronics, optoelectronics, and catalysis. However, fully exploiting the outstanding properties will require a method for their efficient exfoliation. Here we present that a series of layered materials can be successfully exfoliated into single- and few-layer nanosheets using the driving forces coming from the phase inversion, i.e., from micelles to reverse micelles in the emulsion microenvironment built by supercritical carbon dioxide (SC CO2). The effect of variable experimental parameters including CO2 pressure, ethanol/water ratio, and initial concentration of bulk materials on the exfoliation yield have been investigated. Moreover, we demonstrate that the exfoliated 2D nanosheets have their worthwhile applications, for example, graphene can be used to prepare conductive paper, MoS2 can be used as fluorescent label to perform cellular labelling, and BN can effectively reinforce polymers leading to the promising mechanical properties.

Formation of controllable hydrophilic/hydrophobic drug delivery systems by electrospinning of vesicles

Novel multifunctional poly(ethylene oxide) (PEO) nanofibrous membrane, which contains vesicles constructed by mixed surfactant cetyltrimethylammonium bromide (CTAB)/sodium dodecylbenzenesulfonate (SDBS), has been designed as dual drug-delivery system and fabricated via the electrospinning process. 5-FU and paeonolum, which are hydrophilic and hydrophobic anticancer model drugs, can be dissolved in vesicle solution's bond water and lipid bilayer membranes, respectively. The physicochemical properties of the electrospun nanofibrous membrane were systematically studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), and X-ray diffraction (XRD). Drug release behaviors of the electrospun nanofibrous membrane fabricated with different molar ratio of CTAB/SDBS vesicle solution were investigated. The result showed that the releasing amount of hydrophilic drug presented an ascending release manner, while the hydrophobic one showed a descending release behavior with increasing of the molar ratio of CTAB/SDBS. Moreover, the release amount of drugs from drug delivery system can be controlled by the molar ratio of CTAB/SDBS in the vesicle solution easily and conveniently. The distinct properties can be utilized to encapsulate environmental demanding and quantificational materials.

Dual template effect of supercritical CO2 in ionic liquid to fabricate a highly mesoporous cobalt metal–organic framework

Dual template effect of supercritical CO₂ in ionic liquid to fabricate a highly mesoporous cobalt metal–organic framework.

Self-assembly and disassembly of a redox-responsive ferrocene-containing amphiphilic block copolymer for controlled release

The synthesis and self-assembly of ferrocene-containing block copolymers PEG-b-PMAEFc, and the encapsulation and redox-responsive release of a model molecule (rhodamine B) upon external redox stimuli (H₂O₂).

Highly mesoporous metal-organic framework assembled in a switchable solvent

The mesoporous metal-organic frameworks are a family of materials that have pore sizes ranging from 2 to 50 nm, which have shown promising applications in catalysis, adsorption, chemical sensing and so on. The preparation of mesoporous metal-organic frameworks usually needs the supramolecular or cooperative template strategy. Here we report the template-free assembly of mesoporous metal-organic frameworks by using CO2-expanded liquids as switchable solvents. The mesocellular metal-organic frameworks with large mesopores (13-23 nm) are formed, and their porosity properties can be easily adjusted by controlling CO2 pressure. Moreover, the use of CO2 can accelerate the reaction for metal-organic framework formation from metal salt and organic linker due to the viscosity-lowering effect of CO2, and the product can be recovered through CO2 extraction. The as-synthesized mesocellular metal-organic frameworks are highly active in catalysing the aerobic oxidation of benzylic alcohols under mild temperature at atmospheric pressure.

Isolation and selection of new biosurfactant producing bacteria from degraded palm kernel cake under liquid state fermentation

Biosurfactants are surface-active compounds produced by different microorganisms. The aim of this study was to introduce palm kernel cake (PKC) as a novel substrate for biosurfactant production using a potent bacterial strain under liquid state fermentation. This study was primarily based on the isolation and identification of biosurfactant-producing bacteria that could utilize palm kernel cake as a new major substrate. Potential bacterial strains were isolated from degraded PKC and screened for biosurfactant production with the help of the drop collapse assay and by analyzing the surface tension activity. From the screened isolates, a new strain, SM03, showed the best and most consistent results, and was therefore selected as the most potent biosurfactant-producing bacterial strain. The new strain was identified as Providencia alcalifaciens SM03 using the Gen III MicroPlate Biolog Microbial Identification System. The yield of the produced biosurfactant was 8.3 g/L.

Core-shell inversion by pH modulation in dynamic covalent micelles

Dynamic covalent surfactants have been obtained by the reversible condensation of a hydrophobic aldehyde (ended by an ionic tip) with various neutral polyethylene glycol based hydrophilic amines. In water, the duality between the two hydrophilic domains (charged and neutral) leads to their segregation when the surfactants are self-assembled within micelles. Depending on the number of polyethylene glycol units, a core-shell inversion leading to a switching orientation of the ionic tips from the inside to the outside of the micelles has been demonstrated by a combination of scattering techniques. In competition experiments, when several amines of different pKas and hydrophilic polyethylene glycol chains are competing for the same aldehyde, it becomes possible to trigger this core-shell inversion by pH modulation and associated dynamic constitutional reorganization.

CO₂-responsive microemulsions based on reactive ionic liquids

We demonstrate that the nanodomains within a ternary system consisting of oil, surfactant, and a new reactive ionic liquid can be tuned reversibly upon exposure to and removal of CO2 under mild conditions of temperature and pressure. The equilibrium microstructures of these domains have been characterized by small-angle neutron scattering and demonstrate that control over emulsion morphology (and therefore physicochemical properties such as viscosity) and the breaking of emulsions can be achieved without the need for irreversible changes in system composition or significant energy input.

High pressure CO2-controlled reactors: enzymatic chiral resolution in emulsions

Chiral separation of ibuprofen catalyzed by enzyme conducted in CO₂-based micelle makes the reaction more effective and greener.