Reversible Switching of a Micelle-to-Vesicle Transition by Compressed CO2

Synthesis of Atomically Thin g-C3N4 Nanosheets via Supercritical CO2 Doping with Single-Atom Cobalt for Photocatalytic Hydrogen Evolution

The atomically thin nanosheets of graphitic carbon nitride (g-C3N4) with mesopores have been successfully exfoliated with supercritical CO2 (scCO2). The thickness of the as-synthesized samples could be directly tailored by simply regulating the scCO2 pressure. The obtained bilayer mesoporous g-C3N4 nanosheets doped with monatomic Co through a microwave-assisted approach havve been employed as single-atom catalysts to enhance the photocatalytic hydrogen evolution performance. The as-prepared Co/P/CN-sc sample exhibited a boosted H2 production performance due to its unique structural advantages by exposing more active sites and facilitating the separation of charge carriers. Based on X-ray photoelectron spectroscopy, steady-state and time-resolved photoluminescence spectroscopy, X-ray absorption fine structure measurement, and density functional theory (DFT) calculations, a possible mechanism has been proposed. The work shows a new perspective for designing an inexpensive photocatalyst with a unique structure through a facile and green approach for photocatalytic hydrogen evolution.

CO2-Assisted synthesis of hierarchically porous carbon as a supercapacitor electrode and dye adsorbent

A facile and sustainable strategy was developed for the fabrication of hierarchically porous carbons with tunable pore size distributions and architectures.

Gold/Periodic Mesoporous Organosilicas with Controllable Mesostructure by Using Compressed CO2

Gold nanoparticles confined into the walls of periodic mesoporous organosilicas (PMOs) with controllable morphology have been successfully fabricated through a one-pot method by using different CO₂ pressures. The synthesis can be easily conducted in a mixed aqueous solution by using HAuCl₄ as gold source and bis[3-(triethoxysilyl)propyl] tetrasulfide and tetramethoxysilane as the organosilica precursor. P123 and compressed CO₂ served as the template and catalytic/regulative agent, respectively. Transmission electron microscopy, N₂ adsorption, and X-ray diffraction were employed to characterize the structure of the obtained composite materials. To further investigate the formation mechanism, a series of ordered PMOs with one-dimensional nanotube, two-dimensional hexagonal, vesicle-like, and cellular foam structures were obtained by using different CO₂ pressures without the gold source. The mechanism for mesostructure evolution of PMOs with different CO₂ pressures was proposed and discussed in detail. The catalytic performance of Au-based PMOs was evaluated for the reduction of 4-nitrophenol (4-NP). These obtained composites with different mesostructures not only exhibit excellent catalytic activity, high conversion rate, and remarkable thermal stability, but they also exhibit morphology-dependent reaction properties in the reduction of 4-NP. The possible reaction pathway of the reactants to embedded Au active sites was proposed and schemed.

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.

Temperature-induced vesicle to micelle transition in cationic/cationic mixed surfactant systems

Temperature-induced vesicle to micelle transition (VMT), which has rarely been reported in cationic/cationic mixed surfactant systems, was systemically studied in a didodecyldimethylammonium bromide (DDAB)/dodecyltrimethylammonium chloride (DTAC) aqueous solution. We investigated the effect of temperature on DDAB/DTAC aqueous solutions by means of turbidity, conductivity, cryo-TEM, a UV-vis spectrophotometer, and a steady-state fluorescence spectrometer. It was found that increasing temperature could induce the transformation from the vesicle to the micelle in this cationic/cationic mixed surfactant system. The degree of transformation can be easily controlled by the operation temperature. Additionally, by adjusting the proportion of the mixed cationic/cationic systems and employing cationic surfactants with different chain-lengths, we were able to conclude that the hydrophobic tail length of the surfactant affects the aggregation behavior of cationic/cationic mixed surfactant systems as a function of temperature. It is universal to induce the transformation from the vesicle to the micelle by temperature in cationic/cationic mixed surfactant systems. A possible mechanism for the temperature-induced VMT was proposed based on the experimental results.

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.

Dual stimuli-responsive self-assembly transition in zwitterionic/anionic surfactant systems

Temperature and pH responsiveness is important for biological applications in protein reconstitution, gene delivery and controlled drug release. The temperature and pH dual responsive self-assembly transition, vesicle-to-micelle transitions (VMTs) and micelle-to-vesicle transitions (MVTs), in dodecyl sulfonatebetaine (DSB)/sodium bis(2-ethylhexyl) sulfosuccinate (AOT) aqueous solution are investigated. Various experimental techniques including cryogenic transmission electronic microscopy, UV-vis spectroscopy, fluorescence spectroscopy, conductivity, and zeta potential were employed to verify the transformation process. Encapsulation of calcein was further applied in this study. The results showed that the self-assembly transition in DSB/AOT aqueous solution is reversible and can be controlled by temperature and pH. It is anticipated that utilizing simple stimuli methods to realize unique self-assembly behaviour in dilute aqueous solution may offer new possibilities in cancer diagnosis and therapy.

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.

Gold nanoparticles embedded in silica hollow nanospheres induced by compressed CO2 as an efficient catalyst for selective oxidation

Au NPs embedded in the hollow silica were prepared by compressed CO₂ and showed high activity in selective oxidation.

Supramolecular assemblies of amphiphilic L-proline regulated by compressed CO2 as a recyclable organocatalyst for the asymmetric aldol reaction

Compressed CO2 triggers the formation of amphiphilic proline supramolecular assemblies in water, which catalyze the asymmetric aldol reaction without any additives. Compressed CO2 can dynamically regulate the size of the assemblies and subsequently the catalyst activity and selectivity. Furthermore, CO2 provides the merit of easy separation and purification, making the process sustainable and recyclable.

Supercritical or compressed CO2 as a stimulus for tuning surfactant aggregations

Surfactant assemblies have a wide range of applications in areas such as the chemical industry, material science, biology, and enhanced oil recovery. From both theoretical and practical perspectives, researchers have focused on tuning the aggregation behaviors of surfactants. Researchers commonly use solid and liquid compounds such as cosurfactants, acids, salts, and alcohols as stimuli for tuning the aggregation behaviors. However, these additives can present economic and environmental costs and can contaminate or modify the product. Therefore researchers would like to develop effective methods for tuning surfactant aggregation with easily removable, economical, and environmentally benign stimuli. Supercritical or compressed CO(2) is abundant, nontoxic, and nonflammable and can be recycled easily after use. Compressed CO(2) is quite soluble in many liquids, and the solubility depends on pressure and temperature. Therefore researchers can continuously influence the properties of liquid solvents by controlling the pressure or temperature of CO(2). In this Account, we briefly review our recent studies on tuning the aggregation behaviors of surfactants in different media using supercritical or compressed CO(2). Supercritical or compressed CO(2) serves as a versatile regulator of a variety of properties of surfactant assemblies. Using CO(2), we can switch the micellization of surfactants in water, adjust the properties of reverse micelles, enhance the stability of vesicles, and modify the switching transition between different surfactant assemblies. We can also tune the properties of emulsions, induce the formation of nanoemulsions, and construct novel microemulsions. With these CO(2)-responsive surfactant assemblies, we have synthesized functional materials, optimized chemical reaction conditions, and enhanced extraction and separation efficiencies. Compared with the conventional solid or liquid additives, CO(2) shows some obvious advantages as an agent for modifying surfactant aggregation. We can adjust the aggregation behaviors continuously by pressure and can easily remove CO(2) without contaminating the product, and the method is environmentally benign. We can explain the mechanisms for these effects on surfactant aggregation in terms of molecular interactions. These studies expand the areas of colloid and interface science, supercritical fluid science and technology, and chemical thermodynamics. We hope that the work will influence other fundamental and applied research in these areas.

Switching micellization of pluronics in water by CO2

The micellization of amphiphilic molecules is an interesting topic from both theoretical and practical points of view. Herein we have studied the effects of compressed CO(2) on the micellization of Pluronics in water by means of fluorescence, UV/Vis spectra, and small-angle X-ray scattering. It was found that CO(2) can induce the micellization of Pluronics in water, and the micelle can return to the initial state of molecular dispersion after depressurization. Therefore, the micellization of Pluronics in water can be switched through the easy control of pressure. Different from the common micelles with hydrophobic cores, interestingly, this CO(2)-induced micelle has an amphiphilic core, in which hydrophobic and hydrophilic domains coexist. On account of the ability to dissolve both polar and nonpolar components in the micellar core, the CO(2)-induced micelles can improve the reagent compatibilities frequently encountered in various applications. In an attempt to address this advantage, this micelle was utilized as template to the one-step synthesis of Au/silica core-shell composite nanoparticles. Furthermore, the underlying mechanism for the CO(2)-induced micellization of Pluronics in water was investigated by a series of experiments.