Carbon Dioxide Microemulsions Research Articles

Synthesis of Silver and Copper Nanoparticles in a Water-in-Supercritical-Carbon Dioxide Microemulsion

Nanometer-sized silver and copper metal particles can be synthesized by chemical reduction of Ag+ and Cu2+ ions dissolved in the water core of a water in supercritical fluid carbon dioxide microemulsion. Sodium cyanoborohydride and N,N,N‘,N‘-tetramethyl-p-phenylenediamine are effective reducing agents for synthesizing these metal nanoparticles in the microemulsion. Formation of the metal nanoparticles was monitored spectroscopically using a high-pressure fiber-optic reactor equipped with a CCD array UV−vis spectrometer. Silver and copper nanoparticles synthesized in the microemulsion showed characteristic surface plasmon resonance absorption bands centered at 400 and 557 nm, respectively. Diffusion and distribution of the oxidized form of the reducing agent between the micellar core and supercritical CO2 appeared to be the rate-determining step for the formation of the silver nanoparticles in this system.

Preparation of Silver Nanoparticles via Rapid Expansion of Water in Carbon Dioxide Microemulsion into Reductant Solution

Perfluoropolyether ammonium carboxylate stabilized water-in-CO2 microemulsion was used to dissolve silver nitrate salt for the RESOLV (Rapid Expansion of a Supercritical Solution into a Liquid SOLVent) process. The solution of reverse micelles with an aqueous silver nitrate core in CO2 was rapidly expanded into a room-temperature solution of sodium borohydride to form silver nanoparticles. The nanoparticles were characterized using optical, X-ray diffraction, and microscopy techniques. The nanoparticle properties were found to be dependent on the pre-expansion microemulsion conditions in CO2.

Metal Extraction from Heterogeneous Surfaces Using Carbon Dioxide Microemulsions

We have demonstrated that water in carbon dioxide (w/c) microemulsions extract copper and europium ions from a variety of solid substrates in high yields. The microemulsions are unique from bulk aqueous extractions in that the volume of water used is similar to that of the metal extracted, rather than that of the entire solid substrate, allowing grams of waste to be extracted with microliters of water. The microemulsion enhances wetting without saturation of the solid matrix, allowing almost complete recovery of the metal ions. Our results show that >98% of the metal can be recovered from a filter paper surface in a single extrtaction step. Extraction experiments using wood spiked with metal ions have shown that the microemulsions have enhanced diffusivity compared to bulk water. Pressure changes allow recovery and regeneration of the surfactant; in a test case, 81% of the initial capacity is achieved upon reexposure of the w/c microemulsion to a second batch of copper nitrate.

Water in Carbon Dioxide Microemulsions with Fluorinated Analogues of AOT

A series of fluorinated analogues of AOT, the sodium salt of bis(3,3,4,4,5,5,6,5,7,7,8,8,8-tridecafluorooctyl)-2-sulfosuccinate (CF3(CF2)5CH2CH2OOCCH2CH(SO3Na)COOCH2CH2(CF2)5CF3, di-HCF7), the sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate (H(CF2)4CH2OOCCH2CH(SO3Na)COOCH2(CF2)4H, di-HCF4), and the sodium salt of bis(2,2,3,4,4,4-hexafluorobutyl)-2-sulfosuccinate (CF3CHFCF2CH2OOCCH2CH(SO3Na)COOCH2CF2CHFCF3, di-HCF3), were synthesized and characterized by 1HNMR. The pressure−temperature phase diagrams for W/CO2 microemulsions stabilized by the three surfactants were determined. At a fixed temperature, cloud point pressures increased with increasing water-to-surfactant ratio. The water solubilization capacity increased in the order di-HCF4 > di-HCF3 > di-HCF7.

Metal extractions using water in carbon dioxide microemulsions

Water in supercritical carbon dioxide microemulsions are examined as a new medium for the extraction of metal ions from contaminated surfaces, and are shown to extract >99% of the copper from a spiked filter paper with as little as a two-fold excess of surfactant.

Dissolving biomolecules and modifying biomedical implants with supercritical carbon dioxide

Abstract We describe two methodologies for dissolving ionic/polar species in scCO2. Both lead to a broadening of the range of applications for scCO2. Fluorinated surfactants may be used to prepare water in carbon dioxide microemulsions to allow solubilization of ionic and biological species. We outline also the preparation of scCO2 soluble metal precursors that can be impregnated efficiently into polymeric substrates. Further processing by heat or UV light leads to metallic particles distributed throughout a polymer substrate. The clean synthesis of such composites can be applied to the development of improved medical implants.

Synthesizing and Dispersing Silver Nanoparticles in a Water-in-Supercritical Carbon Dioxide Microemulsion.

Reverse micelles and microemulsions formed in liquid and supercritical carbon dioxide (CO{sub 2}) allow highly polar or polarizable compounds to be dispersed in this nonpolar fluid. However, since the polarizability per unit volume of dense CO{sub 2} is quite low, it is difficult to overcome the strong van der Waals attractive interactions between particles in order to stably suspend macromolecular species. Conventional surfactants by themselves do not form reverse micelles or microemulsions in CO{sub 2} because the van der Waals interdroplet attractions are too high. The use of surfactants or cosurfactants with fluorinated tails provides a layer of a weakly attractive compound covering the highly attractive droplet cores, thus preventing their short-range interactions that would destabilize the system. Using this strategy, the authors describe a method to synthesize and stabilize metallic silver nanoparticles having diameters from 5 to 15 nm in supercritical CO{sub 2} using an optically transparent, water-in-CO{sub 2} microemulsion.

Water in Supercritical Carbon Dioxide Microemulsions: Spectroscopic Investigation of a New Environment for Aqueous Inorganic Chemistry

In this paper, we present spectroscopic evidence for the formation of water in supercritical carbon dioxide (scCO2) microemulsions stabilized by an ammonium carboxylate perfluoro polyether (PFPE) surfactant. FTIR spectroscopy has been employed to determine the existence of “bulk” (hydrogen-bonded) water (H2O and D2O) at the core of the microemulsions, and to distinguish between this and the presence of “free” (monomeric) water dissolved in the scCO2 and builds on preliminary results described elsewhere (Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624). Cloud point studies confirm that optically transparent and thermodynamically stable microemulsion solutions are formed. We have investigated the utility of these microemulsions as novel environments for reaction chemistry. In particular, we have shown that, using the PFPE surfactant, an aqueous solution of potassium permanganate (KMnO4) may be dispersed in scCO2, leading to a purple-colored solution with concentration on the order of 5 × 10-4 M, as detected by UV−vis absorption spectroscopy. Moreover, aqueous sodium nitroprusside (Na2[Fe(CN)5(NO)]) and potassium dichromate (K2Cr2O7) are also shown to be soluble in the water/PFPE/scCO2 microemulsions and to undergo simple aqueous inorganic reactions with gaseous reactants such as H2S and SO2. Methyl orange has been used to investigate the presence of a carbonic acid microenvironment in the water/PFPE/scCO2 microemulsions.

A Small-Angle Neutron Scattering Study of Water in Carbon Dioxide Microemulsions

Phase behavior and small angle neutron scattering (SANS) experiments were performed on mixtures of D2O/CO2/ammonium carboxylate perfluoropolyether (PFPE) surfactant as a function of pressure (192−287 bar) and D2O composition (0.8−2.0 wt %) at 35 °C. SANS measurements provided direct evidence of water-swollen inverted micelles in CO2. At a constant PFPE concentration of 2.1 wt % in CO2, the microemulsion core radius increased from 20 to 36 Å as the D2O concentration increased from 0.8 to 2.0 wt %. For a constant water concentration, parameters extracted from model fits of the SANS spectra indicated that as the phase boundary was approached on reducing pressure, micellar structure remained essentially unchanged, while critical fluctuations increased.