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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 CO(2) 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 CO(2) could induce the MVT in the surfactant system, and the vesicles returned to the micelles simply by depressurization; that is, CO(2) 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.

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.

The effect of compressed CO 2 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 CO 2 pressures were also determined. The solubilization capacity of the reverse micelles for the protein increased considerably as CO 2 pressure increased within the low-pressure range, but decreased at higher CO 2 pressures, so that the micelles eventually lost their ability to solubilize the protein. The effect of CO 2 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 CO 2 in the extraction of proteins with reverse micelles.

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.

The effects of two polar co-solvents, chlorodifluoromethane and acetone, on the solubility and enthalpy of a solution of 1,4-naphthoquinone in supercritical (SC) CO2 were studied. We found that the dissolution process becomes less exothermic in the presence of the co-solvents relative to that in pure CO2, although the solubility is enhanced significantly by the co-solvents. This indicates that the increase in the solubility by adding co-solvents results from the increase of the entropy of solution. On the basis of the unexpected results we propose a new mechanism for the solubility enhancement of the solute by the co-solvents in supercritical fluids (SCF); this should be applicable to cases in which the local density of the SC solvent around the solute and the co-solvent is larger, and the co-solvent associates preferentially with the solute. The results are also very important for the understanding of other fundamental questions of SCF science, such as the effect of co-solvents on the thermodynamic and kinetic properties of the reactions in SCFs.

Silver nanoparticles weresynthesizedinsodiumbis(2-ethylhexyl)-sulfosuccinate(AOT)reversemicellesin isooctane with tetraethylene glycoldodecylether(C 12 E 4 )asacosurfactant.RecoveryoftheAgparticlesfromthereverse micelles by dissolving antisol-ventCO 2 inthemicellarsolutionwasinvestigated.AlltheAgparticlesinthereversemicellescouldbeprecipitatedbycompressedCO 2 atsuitablepressures,whilethesurfactantsremainedintheisooctanecontinuousphase,andwell-dispersed Ag nanoparticles were ob-tained.Theeffectsofoperatingcondi-tionsonthesizeandsizedistributionoftheAgparticleswereinvestigated.Theparticlesizedecreasedwithdecreasingmolarratio(w)ofwatertosurfactant.Ahigher CO 2 pressure in the recoveryprocessfavoredproductionofsmallerparticles.AdecreaseinthemolarratioofreductantKBH 4 toAgNO 3 resultedinlargerAgparticleswithhigherpolydis-persity.Keywords: carbon dioxide ¥micelles ¥nanostructures silversurfactants Introduction Many efforts have been devoted to the preparation ofnanoparticles,anewlyemergingfieldinsolid-statechemistry,andtostudyingtheirproperties.