Enhanced solubilization of bovine serum albumin in reverse micelles by compressed CO2

The effect of compressed CO2 on the solubilization of bovine serum albumin (BSA) in water/sodium bis-(2-ethylhexyl) sulfosuccinate (AOT)/isooctane reverse micelles was studied by observing phase behavior and recording UV-visible spectra under different conditions. The pH values within the water cores of reverse micelles at different CO2 pressures were also determined. The solubilization capacity of the reverse micelles for the protein increased considerably as CO2 pressure increased within the low-pressure range, but decreased at higher CO2 pressures, so that the micelles eventually lost their ability to solubilize the protein. The effect of CO2 on the stability of the reverse micelles played an important role in the relationship between pressure and protein solubility. A "multicomplex" model was proposed to explain these effects. The different solublization capacities within different pressure ranges demonstrates the unique advantage of using compressed CO2 in the extraction of proteins with reverse micelles.     

Effect of compressed CO2 on the critical micelle concentration and aggregation number of AOT reverse micelles in isooctane

The effect of compressed CO2 on the critical micelle concentration (cmc) and aggregation number of sodium bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles in isooctane solution was studied by UV/Vis and fluorescence spectroscopy methods in the temperature range of 303.2-318.2 K and at different pressures or mole fractions of CO2 (X(CO2)). The capacity of the reverse micelles to solubilize water was also determined by direct observation. The standard Gibbs free energy (DeltaGo(m)), standard enthalpy (DeltaHo(m)), and standard entropy (DeltaSo(m)) for the formation of the reverse micelles were calculated by using the cmc data determined. It was discovered that the cmc versus X(CO2) curve and the DeltaGo(m) versus X(CO2) curve for a fixed temperature have a minimum, and the aggregation number and water-solubilization capacity of the reverse micelles reach a maximum at the X(CO2) value corresponding to that minimum. These results indicate that CO2 at a suitable concentration favors the formation of and can stabilize AOT reverse micelles. A detailed thermodynamic study showed that the driving force for the formation of the reverse micelles is entropy.     

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

The study of the micelle‐to‐vesicle transition (MVT) is of great importance from both theoretical and practical points of view. Herein, we studied the effect of compressed CO₂ on the aggregation behavior of dodecyltrimethylammonium bromide (DTAB)/sodium dodecyl sulfate (SDS) mixed surfactants in aqueous solution by means of direct observation, turbidity and conductivity measurements, steady‐state fluorescence, time‐resolved fluorescence quenching (TRFQ), fluorescence quantum yield, and template methods. Interestingly, all these approaches showed that compressed CO₂ could induce the MVT in the surfactant system, and the vesicles returned to the micelles simply by depressurization; that is, CO₂ can be used to switch the MVT reversibly by controlling pressure. Some other gases, such as methane, ethylene, and ethane, could also induce the MVT of the surfactant solution. A possible mechanism is proposed on the basis of the packing‐parameter theory and thermodynamic principles. It is shown that the mechanism of the MVT induced by a nonpolar gas is different from the MVT induced by polar and electrolyte additives.     

A novel method to synthesize polystyrene nanospheres immobilized with silver nanoparticles by using compressed CO2

In this work, a novel route to synthesize polymer/metal composite nanospheres has been proposed. This method combines the advantages that the polymer chains collapse and entangle in the presence of compressed CO(2), which acts as antisolvent, and the metal nanoparticles and polymers can be precipitated simultaneously from micellar solutions by the easy control of CO(2) pressure. Ag/polystyrene (PS) nanocomposites have been successfully prepared using this method. The transmission electronic micrographs (TEM) of the obtained nanocomposites show that the smaller Ag nanoparticles are immobilized by PS nanospheres of about 50 nm; the phase structure was characterized by X-ray diffraction (XRD). The Ag/PS nanocomposites show absorption properties at a wavelength of approximately 417 nm. The results of X-ray photoelectron spectra (XPS) and FT-IR spectra indicate that there is no chemical linkage or strong interaction between PS and Ag nanoparticles in the resultant products. This method has many potential advantages for applications and may easily be applied to the preparation of a range of inorganic/ polymer composite nanoparticles.     

Enhancement of the solubilization capacity of water in Triton X-100/cyclohexane/water system by compressed gases

The effect of compressed CO2 and ethylene on the properties of Triton X-100/cyclohexane/water systems was studied at different temperatures and pressures. Surprisingly, it was discovered that the compressed gases had the functions of co-surfactants. At suitable pressures, the water-to-surfactant molar ratio (W0) was enhanced significantly by the dissolution of the gas in the solution. The microenvironment in the reverse micelles was investigated by UV-visible spectroscopy by using methyl orange (MO) as a probe. The influence of n-hexane, Na2CO3, NaHCO3, H2C2O4, and CaCl2 at various concentrations on the solubilization of water in the absence of compressed gases was also investigated in order to obtain some information about the mechanism of the interesting phenomenon. This new route to stabilize reverse micelles may have potential applications to other similar systems. Moreover, the results of this work provide some useful information to get insight into the mechanism of co-surfactants, because a conventional co-surfactant usually contains both polar group and hydrocarbon chain, and it is very difficult to clarify the functions these two groups, while the gases used in this work are small nonpolar molecules, which solely have the function of the hydrocarbon chain in a co-surfactant.     

Kinetics of horseradish peroxidase reactions in aqueous and reverse micelles composed of nonionic, polyoxyethylene alkyl ether surfactants

Differences between the rates of oxidation of substrates by horseradish peroxidase in buffer, aqueous micellar solution of Brij 35 and in reverse micelles formed of Brij 30 in several hydrocarbons are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of compressed CO2 on the properties of lecithin reverse micelles

Lecithin is a very useful biosurfactant. In this work, the effects of compressed CO 2 on the critical micelle concentration (cmc) of lecithin in cyclohexane and solubilization of water, lysozyme, and PdCl 2 in the lecithin reverse micelles were studied. The micropolarity and pH value of the polar cores of the reverse micelles with and without CO 2 were also investigated. It was found that CO 2 could reduce the cmc of the micellar solution and enhance the capacity of the reverse micelles to solubilize water, the biomolecule, and the inorganic salt significantly. Moreover, the water pools could not be formed in the reverse micelles in the absence of CO 2 because of the limited amount of water solubilized. However, the water pools could be formed in the presence of CO 2 because large amounts of water could be solubilized. All of these provide more opportunity for effective utilization of this green surfactant. The possible mechanism for tuning the properties of the reverse micelles by CO 2 is discussed.     

Enhancement of Formation of Polar Microenvironment in Polymer by Compressed CO_2

To date, polar microenvironments in apolar solvents have been successfully used in different ways, such as separation of proteins1, enzymatic or catalytic reactions in reverse micelles2. It is obvious that investigation of new method to create polar microenvironments is of great importance to both pure and applied sciences, and it is desirable that forming and breaking the microenvironments can be easily controlled. Compressed CO2 can dissolve in many organic solvents and the solubility can …     

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.     

Tri‐Chain Hydrocarbon Surfactants as Designed Micellar Modifiers for Supercritical CO2

Getting their feet wet : Low‐cost hydrocarbon surfactants act as fluid modifiers for supercritical carbon dioxide (scCO₂). Increased terminal branching of the surfactant chains aids micelle formation (see middle picture: CO₂ green), and more chains allows water to be incorporated (right, blue).

     

An Artificial Electrode/Electrolyte Interface for CO2 Electroreduction by Cation Surfactant Self-Assembly

In this work, an artificial electrode/electrolyte (E/E) interface, made by coating the electrode surface with a quaternary ammonium cation (R₄N⁺) surfactant, was successfully developed, leading to a change in the CO₂ reduction reaction (CO₂RR) pathway. This artificial E/E interface, with high CO₂ permeability, promotes CO₂ transportation and hydrogenation, as well as suppresses the hydrogen evolution reaction (HER). Linear and branched surfactants facilitated formic acid and CO production, respectively. Molecular dynamics simulations show that the artificial interface provided a facile CO₂ diffusion pathway. Moreover, density-functional theory (DFT) calculations revealed the stabilization of the key intermediate, OCHO*, through interactions with R₄N⁺. This strategy might also be applicable to other electrocatalytic reactions where gas consumption is involved.     

Inhibited phenol ionization in reverse micelles: confinement effect at the nanometer scale

We found that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP), and p-nitrophenol (p-NPh) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane reverse micelles (RMs) at various W(0) (W(0) = [H(2)O]/[surfactant]) values studied changed with time if (-)OH ions were present in the RM water pool. There is an evolution of ionized phenol (phenolate) bands to nonionized phenol absorption bands with time and this process is faster at low W(0) values and with phenols with higher bulk water pK(a) values. That is, in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, whereas in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert into nonionized phenol species over time. Furthermore, we demonstrate that, independent of the (-)OH concentration used to prepare the AOT RMs, the nonionized phenols are the more stable species in the RM media. We explain our results by considering that strong hydrogen-bonding interactions between phenols and the AOT polar head groups result in the existence of only nonionized phenols at the AOT RM interface. The situation is quite different when the phenols are dissolved in cationic benzyl-n-hexadecyldimethylammonium chloride RMs. Therein, only phenolates species are present at the (-)OH concentrations used. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment, such as in the interior of RMs and challenge the general idea that pH can be determined inside RMs.     

Effect of compressed CO2 on the chloroperoxidase catalyzed halogenation of 1,3-dihydroxybenzene in reverse micelles

The effect of compressed CO2 on the specific activity of chloroperoxidase (CPO) to catalyze the chlorination of 1,3-dihydroxybenzene in cetyltrimethylammonium chloride (CTAC)/H2O/octane/pentanol reverse micellar solution was studied. The results show that the specific activity of the enzyme can be enhanced significantly by compressed CO2, and the specific activity can be tuned continuously by changing pressure. The mechanism for the specific activity enhancement of the enzyme by CO2 was also studied. We believe that compressed CO2 can be utilized to tune some other enzyme catalytic reactions in different reverse micellar systems with potential advantages.     

Construction of Versatile and Functional Nanostructures Derived from CO2‐based Polycarbonates

The construction of amphiphilic polycarbonates through epoxides/CO₂ coupling is a challenging aim to provide more diverse CO₂‐based functional materials. In this report, we demonstrate the facile preparation of diverse and functional nanoparticles derived from a CO₂‐based triblock polycarbonate system. By the judicious use of water as chain‐transfer reagent in the propylene oxide/CO₂ polymerization, poly(propylene carbonate (PPC) diols are successfully produced and serve as macroinitiators in the subsequent allyl glycidyl ether/CO₂ coupling reaction. The resulting ABA triblock polycarbonate can be further functionalized with various thiols by radical mediated thiol–ene click chemistry, followed by self‐assembly in deionized water to construct a versatile and functional nanostructure system. This class of amphiphilic polycarbonates could embody a powerful platform for biomedical applications.     

Efficient CO2 capturer derived from as-synthesized MCM-41 modified with amine

A new strategy to synthesize a highly efficient CO(2) capturer by incorporating tetraethylenepentamine (TEPA) into as-synthesized MCM-41 (AM) is reported. The amine guest can be distributed in the micelle of the support, forming a web within the mesopore to trap CO(2) molecules and resulting in a high adsorption capacity for CO(2) up to 237 mg g(-1). Four samples of the as-synthesized MCM-41 with a different amount or type of surfactant are employed as supports to investigate the influence of micelles on the CO(2) adsorption, and the spokelike structure of the micelle in the channel of the support is proven to be essential to the distribution of guest amine. Among these supports, the AM sample is the most competitive due to the advantages of energy and time saving in preparation of the support along with the resulting higher CO(2) adsorption capacity. At the optimal loading of 50 wt % TEPA, the AM-50 sample exhibits a high adsorption capacity of 183 mg g(-1) in the sixth adsorption cycle at 5 % CO(2) concentration.     

Stability of invertase in reverse micelles

The stability of invertase was studied under various conditions, including at 75°C, in presence of stabilizers (sorbitol and glycerol) at 75°C, and in the presence of denaturants (urea and trichloroacetic acid) at 37°C in reverse micelles. Stability of the invertase in reverse micelles was found to be improved over that of the enzyme in bulk aqueous solution. Sorbitol could enhance enzyme stability as it does in the bulk aqueous system. The stabilizing effect of glycerol was reduced in reverse micelles. The denaturation pattern of urea remains unaltered. However, the denaturation effect of trichloroacetic acid has been reduced in reverse micelles.     

Solvolysis of Tris-p-nitrophenyl-phosphate in aqueous and reverse micelles

The widespread use of toxic phosphates and phosphonates as insecticides, and their use as chemical weapons, has led to investigation of fast detoxification and decontamination methods. Micelles, microemulsions, cyclodextrines and liposomes have been used to accelerate phosphate ester decomposition by nucleophiles. Here, hydrolysis, methanolysis and hexanolysis of Tris-p-nitrophenyl phosphate (TNPP), a model for reactive phosphate esters, were studied in homogeneous phase, aqueous and reverse micelles. Kinetic micellar effects were quantitatively analyzed using pseudo-phase models. TNPP hydrolysis was catalyzed by cetyltrimethylammonium chloride (CTAC), cetyltrimethylammonium bromide (CTAB), and hexadecylammonium propanesulfonate (HPS), micelles by factors of five, CTAC, and three, CTAB, HPS, respectively. The calculated rate constants for spontaneous and acetate-catalyzed hydrolysis in the micellar phase were significantly higher than those in the aqueous phase. While in water and in methanol the effect of the acetate cation was negligible, the catalytic efficiency of acetate for hexanolysis depended on the nature of the cation with the K⁺ salt being ca. 20 times more efficient than the tetraethylammonium salt in non-polar solvents. Sodium dodecylsulfate, SDS, micelles inhibited TNPP hydrolysis by a factor of eigth. Reverse micelles of CTAB in n-hexanol/isooctane (10:90, v/v) did not catalyze TNPP hydrolysis, but changed the bis-p-nitrophenyl phosphate/hexyl-bis-p-nitrophenylphosphate product ratio depending of CTAB concentration and water/detergent ratio.