Micelles In Supercritical Carbon Dioxide Research Articles

Optimal aggregation number of reverse micelles in supercritical carbon dioxide: a theoretical perspective.

The aggregation number is one of the most fundamental and important structural parameters for the micelle or reverse micelle (RM) system. In this work, a simple, reliable method for the determination of the aggregation number of RMs in supercritical CO2 (scCO2) was presented through a molecular dynamics simulation. The process of pulling surfactants out of the RMs one by one was performed to calculate the aggregation number. The free energies of RMs with different numbers of surfactants were calculated through this process. We found an RM with the lowest free energy, which was considered to have the optimal number of surfactants. Therefore, the optimal aggregation number of RMs was acquired. In order to explain the existence of an optimal aggregation number, detailed analyses of surfactant accumulation were conducted by combining molecular dynamics with quantum chemistry methods. The results indicated that in the RMs with the lowest free energy, the head-group and tail-terminal of the surfactants accumulated on an equipotential surface. In this case, the surfactant film could effectively separate water and CO2; thus, the lowest free energy was expected. This method determined the aggregation number of RMs by theoretical calculations that did not depend on experimental measurements. This presented approach facilitates the evaluation of the characteristics of RMs in scCO2 and can be further applied in the RM system of organic solvents or even in the micellar system.

Effects of Reverse Micelles-Complexation-Supercritical CO<sub>2</sub> Extraction Conditions on Removal of Pb, Hg and as Ions in Lotus Leaf

Removal rates of Pb, Hg and As ions in lotus leaf extracts obtained by Micelles- complexation-supercritical CO2 extraction through an orthogonal experiment(L16(45)) were investigated to get the best extraction conditions. The results of orthogonal experiments indicated that the removal rate of Pb, Hg and As ions trace heavy metal ions is high by complexaction and reverse micelles in supercritical carbon dioxide (Sc-CO2), and the removal rate of Pb, Hg, As from lotus leaf is 87.85%, 89.19% and 84.72% respectively with the loss rate of total flavonoids is 8.73% under the potential optimum conditions which are 50°C, 20 MPa, 1.5 h, the concentration of Triton X-100 of 0.15 mol.L-1, the mass of complexing agent is 0.1 g.

High-pressure real-time 129Xe NMR: monitoring of surfactant conformation during the self-assembly of reverse micelles in supercritical carbon dioxide

High-pressure (129)Xe NMR has been used to monitor in real time the self-assembly of reverse water-in-supercritical carbon dioxide micelles in the absence of shear, without the need for sensitivity enhancement techniques such as hyperpolarization.

Preparation of Conducting Polyaniline and Polyaniline- Fluorinated Montmorillonite Nanocomposites in Supercritical Carbon Dioxide

A new polymerization procedure with the aid of reverse micelles in supercritical carbon dioxide (SCCO2) has been employed to synthesize polyaniline (PANI). Sodium bis(2-ethylhexyl)-sulfosuccinate (AOT) and polyether-modified polysiloxane (PeSi) were used as mixing surfactants. PANI with more favorable needle-like aggregation structure, higher crystallinity, more excellent conductivity (7.22 S cm− 1) and better dispersibility in ethanol has been obtained via the inverse emulsion polymerization in SCCO2, compared to the PANI synthesized in isooctane. These results were demonstrated by the analysis of Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), X-ray diffraction patterns (XRD) and conductivity measurement. The new polymerization procedure was also employed to prepare polyaniline-fluorinated montmorillonite (PANI-FMMT) nanocomposites in SCCO2. The nanocomposites with highly concentrated (12 wt%–50 wt% loading to monomer), fully delaminated FMMT were successfully prepared. The intercalation of FMMT in SCCO2 was characterized by FTIR, XRD, TEM. Thermogravimetry Analysis (TGA) was performed to investigate the enhancement of thermal stability of PANI-FMMT nanocomposites. SCCO2 facilitate the synthesis of PANI with high performance and PANI-MMT with high MMT concentration. SCCO2 is not only an “alternative” solvent for its environmental-friendly characteristic, but also a more “effective” medium compared to some organic solvent (such as isooctane) to prepare PANI and its intercalated composite materials.

Self-Assembled Reverse Micelles in Supercritical CO2 Entrap Protein in Native State

Molecular dynamics simulations of random quaternary mixtures of protein-water-CO2-fluorosurfactants show the self-assembly of reverse micelles in supercritical carbon dioxide where the protein becomes entrapped inside the aqueous pool. Analyses show that the protein native state remains intact in the water pool. This is because of the bulk nature of the enclosed water that provides a suitable environment for the extracted protein. Results from ab initio calculations imply that the existing fluorosurfactants can be made more effective in stabilizing water-in-CO2 microemulsions by a partial hydrogenation in their tails. A Lewis acid-Lewis base interaction among CO2 and the surfactant tails enhances the stability of the aqueous droplets substantially. The study can help accelerate the search for surfactant process for environmentally benign applications in dense CO2.

Formation of reverse micelles in supercritical carbon dioxide and its thermodynamics

The solubilization behavior of methyl orange as a solvation probe in multiple systems composed of supercritical carbon dioxide, surfactants and co-solvents, is studied. It is concluded that some surfactants, such as sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) and isooctyl phenol polyethoxylate (TX-10), could form reverse micelles in supercritical carbon dioxide under the action of butanol. The formation of reverse micelles is a spontaneous process thermodynamically. Specifically for the nonionic surfactant TX-10, the formation of reverse micelles is dependent on the entropy increase in the system, while for the anionic surfactant AOT, the micellization is mainly dominated by the increase in enthalpy at higher temperatures, but by the increase in entropy at lower temperatures.

Micelle-hosted bimetallic Pd–Ru nanoparticle for in situ catalytic hydrogenation in supercritical CO 2

Bimetallic Pd–Ru nanoparticles of different elemental ratios are prepared via in situ reduction of their simple salts in reverse micelles in supercritical carbon dioxide (scCO 2). The optimised Pd:Ru (1:1) nanoparticle shows the highest activity for hydrogenation of functionalised alkene under mild conditions, which can be easily recycled under the reaction conditions without use of organic solvent.

Molecular dynamics simulation of a reverse micelle self assembly in supercritical CO2.

In this communication we report on molecular dynamics computer simulations of self-assembly of reverse micelles in supercritical carbon dioxide. The reverse micelles contain perfluoropolyether ammonium carboxylate surfactants and an aqueous core. We observed a quick self-assembly of these micelles over time periods of approximately 5 ns, irrespective of initial conditions. In most cases, the self-assembled perfluorinated reverse micelles have a nice spherical shape and properties consistent with experiments. When the fluorinated surfactant is replaced by its hydrogenated analogue, the assembled aggregate contains a region of direct contact between water and carbon dioxide, indicating that hydrogenated surfactant is not a good agent for creation of microemulsions in water/carbon dioxide mixtures.

Dyeing of natural fibres from perfluoro‐polyether reverse micelles in supercritical carbon dioxide

The dyeing of natural fibres from a reverse‐micellar system in supercritical carbon dioxide has been investigated using ammonium carboxylate perfluoropolyether as surfactant. Protein fibres such as silk and wool were satisfactorily dyed in deep shades with conventional acid dyes without special pretreatment. Exhaustion was excellent. It was however found that on these fibres the performance of acid dyes was strongly influenced by temperature and carbon dioxide density. Conventional reactive dyes in this system were adsorbed on cotton, even in the absence of dyeing auxiliaries, but the fixation of the dye was not satisfactory. Compared with supercritical dyeing methods reported previously, it was found in this study that the temperature and pressure required were significantly lower.

Molecular Dynamics Simulation Studies of Polyether and Perfluoropolyether Surfactant Based Reverse Micelles in Supercritical Carbon Dioxide

Molecular dynamics simulations are performed to investigate the structural properties of aqueous reverse micelles in supercritical carbon dioxide. A carboxylate perfluoropolyether is used as the surfactant in the first simulation. The results show that the micelle remains stable over a 5 ns time period. Calculated micellar properties are found to be in good agreement with the available experimental data. Two other simulations are performed on systems where all fluorine atoms of the surfactant molecules are replaced with hydrogen atoms. In the first of these simulations the number of molecules is the same as in the simulation with perfluorinated surfactant. In the second simulation the exposure of an aqueous core to carbon dioxide is similar to the one observed in the simulation with perfluorinated surfactant. We observe that fluorinated chains coordinate more carbon dioxide molecules compared to the pure polyether chains. This has a direct influence on the micellar structure.

Structure of Phosphate Fluorosurfactant Based Reverse Micelles in Supercritical Carbon Dioxide

The existence of microemulsions in the system composed of phosphate-based fluorosurfactant, water, and supercritical CO2 is demonstrated by small-angle neutron scattering experiments. A computer simulation study performed on a reverse micelle created in this system shows that the micelle remains stable over a 4 ns time period of the simulation. While the data obtained from the experiments provide information about the size of the reverse micelle, the data obtained from the simulations provide additional information about the shape and structure of such a micelle.

Electrical conductivity of reverse micelles in supercritical carbon dioxide

The conductivity of water/ClPFPE-NH4/scCO2 systems is studied over a wide range of pressures and droplet volume fractions at 35 °C and at two constant water-to-surfactant molar ratios, W0 = 10.3 and 18.7, respectively. The results are interpreted according to three different models for low, intermediate and high concentrations of water and surfactant. Information is obtained about a possible bulk-like behaviour of water, about the approximate size of the micelles, about micellar percolation, and about changes of CO2–water droplet interactions with pressure.

Molecular Dynamics Simulation of Reverse Micelles in Supercritical Carbon Dioxide

We have studied surfactants and reverse micelles in supercritical carbon dioxide (SCO2) by molecular dynamics simulation using molecular potential models that we have developed for this purpose. In this work we present simulation results suggesting how varying the surfactant molecular architecture and chemistry may affect the structure and properties of reverse micelles in SCO2. We focus on a dichain surfactant, (C7H15)(C7F15)CHSO4-Na+, which forms reverse micelles with aqueous cores in SCO2.

The Morphology of Block Copolymer Micelles in Supercritical Carbon Dioxide by Small-Angle Neutron and X-ray Scattering

Above its critical point, carbon dioxide forms a super-critical fluid, which promises to be an environmentally responsible replacement for the organic solvents traditionally used in polymerizations. Many lipophilic polymers such as polystyrene (PS) are insoluble in CO2, though polymerizations may be accomplished via the use of PS-fluoropolymer stabilizers, which act as emulsifying agents. Small-angle neutron and X-ray scattering have been used to show that these molecules form micelles with a CO2-phobic PS core and a CO2-philic fluoropolymer corona. When the PS block was fixed in length and the fluorinated corona block was varied, the number of block copolymer molecules per micelle (six to seven) remained constant. Thus, the coronal block molecular weight exerts negligible influence on the aggregation number, in accordance with the theoretical predictions of Halperin, Tirrell & Lodge [Adv. Polym. Sci. (1992), 100, 31–46]. These observations are relevant to understanding the mechanisms of micellization and solubilization in supercritical fluids.

Use of Dioctylsulphosuccinate Sodium Salt in Supercritical Fluid Chromatography.

Ion-pair reagents and reversed micelles in supercritical carbon dioxide (SF-CO2) were tested as polar mobile phases for supercritical fluid chromatography (SFC). Dioctylsulphosuccinate sodium salt (DSS) was used as the counter-ion in ion-pair supercritical fluid chromatography (IP-SFC) with a packed silica-gel column. DSS/pentane reversed micelles were used in reversed micelle supercritical fluid chromatography (RM-SFC) with a packed fluoroalkyl silica-gel column. IP-SFC and RM-SFC using DSS was demonstrated for analytical separation of ephedrine alkaloids.

Design of Nonionic Surfactants for Supercritical Carbon Dioxide

Interfacially active block copolymer amphiphiles have been synthesized and their self-assembly into micelles in supercritical carbon dioxide (CO2) has been demonstrated with small-angle neutron scattering (SANS). These materials establish the design criteria for molecularly engineered surfactants that can stabilize and disperse otherwise insoluble matter into a CO2 continuous phase. Polystyrene-b-poly(1,1-dihydroperfluorooctyl acrylate) copolymers self-assembled into polydisperse core-shell-type micelles as a result of the disparate solubility characteristics of the different block segments in CO2. These nonionic surfactants for CO2 were shown by SANS to be capable of emulsifying up to 20 percent by weight of a CO2-insoluble hydrocarbon into CO2. This result demonstrates the efficacy of surfactant-modified CO2 in reducing the large volumes of organic and halogenated solvent waste streams released into our environment by solvent-intensive manufacturing and process industries.