Water-in-CO2 microemulsion

Holmes et al. reported the first enzyme catalyzed reactions in water-in-CO2 microemulsions (67). Two reactions, a lipase-catalyzed hydrolysis and a lipoxygenase-catalyzed peroxidation, were demonstrated in water-in-C02 microemulsions using the surfactant di(l/7,l/7,5/7-octafluoro- -pentyl) sodium sulfosuccinate (di-HCF4). A major concern of enzymatic reactions in CO2 is the pH of the aqueous phase, which is approximately 3 when there is contact with CO2 at elevated pressures. Holmes et al. examined the ability of various buffers to maintain the pH of the aqueous solution in contact with CO2. The biological buffer 2-(A-morpholino)ethanesulfonic acid sodium salt (MES) was the most effective, able to maintain a pH of 5, depending on the pressure, temperature, and buffer concentration. The activity of the enzymes in the water-in-C02 microemulsions was comparable to that in a water-in-heptane microemulsion stabilized by the surfactant AOT, which contains the same head group as di-HCF4. 

Holmes JD, Steytler DC, Rees GD, Robinson BH. Bioconversions in a water-in-CO2 microemulsion. Langmuir 1998 14 6371-6376. 

Reverse micelles and microemulsions formed in supercritical CO2 allow highly polar conq>ounds and electrolytes to be dispersed in the non-polar fluid phase. Searching for C02-soluble s actants that would form stable water-in-CO2 microemulsions started a decade ago. A review of the design and performance of surfactants for making stable water-in-C02 microemulsions with all relevant references is presented by Eastoe in Chiq>ter 19. 

In Science, Feb. 1996, E. Goldbaum published a report on the development of a team of scientists of the University of Texas, the University of Nottingham and the University of Colorado, using water in CO2 microemulsions with fluorinated surfactants in place of conventional solvents, such as chlorinated hydrocarbons or hydrocarbons. This research was funded by a Department of Energy grant of the US. 

A high-pressure system used for studying the stability of the water-in-CO2 microemulsions in our laboratory is illustrated in Figure 10. The system consists of a high-pressure cell (20-ml volume) with sapphire windows. The... 

The water-in-C02 microemulsion mentioned previously in this section may provide an effective medium for generating electrical conductivity in supercritical CO2. In 2000, Ohde et al. first reported the results of voltammetric measurements for the redox reactions of ferrocene (FC) and A,fV,iV fV tetramethyl-jc-phenylenediamine (TMPD) in supercritical CO2 in the presence of a water-in-CO2 microemulsion (14). The design of their high-pressure electrochemical cell is shown in Figure 16. The same AOT/PFPE-PO4 water-in-C02 microemulsion described in Section IV.A was used in their voltammetric experiments. Well-defined voltammetric waves were obtained for FC and for TMPD in the microemulsion system as shown in Figure 17. An obvious diffusion current for the redox reaction of FC or TMPD was observed. An electrolysis experiment was also performed with TMPD. After the electrolysis at +0.3 V, the UV-Vis absorption spectrum of the sample collected in hexane was measured. The absorption peak wavelength and the shape of the peak were identical to that for TMPD + in water. The result suggests that TMPD " produced at the electrode surface was in the water core of the water-in-C02 microemulsion, as shown in Fq. (12) ... 

In this work, it was found that the surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600m /g by using the preparation method which we have developed using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. By using these catalysts, it was also found that turnover frequencies for CO2 hydrogenation increased linearly with the catalyst surface area. 

Thermodynamically stable microemulsions and kinetically stable emulsions may be utilized to bring water and nonvolatile hydrophilic substances, such as proteins, ions, and catalysts, into contact with a SCF-continuous phase (e.g. CO2) for separation, reaction and materials formation processes. Reactions between hydrophilic and hydrophobic substrates may be accomplished in these colloids without requiring toxic organic solvents or phase transfer catalysts. CO2 and aqueous phases may be mixed together over a wide range in composition in w/c and c/w emulsions. The emulsion is easily broken by decreasing the pressure to separate the water and CO2 phases, facilitating product recovery and CO2 recycle. Reaction rates can be enhanced due to the considerably lower microviscosity in a w/c as compared to a water-in-alkane microemulsion or emulsion. 

A clear correlation has been observed between limiting surface tension ycmc and surfactant performance in water-in-C02 microemulsions, as measured by the phase transition pressure Ptnms- These results have important implications for the rational design of C02-philic surfactants. Studies of aqueous solutions are relatively easy to carry out, and surface tension measurements can be used to screen target compounds expected to exhibit enhanced activity in CO2. Therefore, potential surfactant candidates can be identified before making time-consuming phase stability measurements in high-pressure CO2. 

According to Zielinski et al, the PFPE-NH4-stabilized water-in-COj microemulsion of Wo equal to 11 contains water droplets of - 4 nm in average diameter, and the droplets and droplet structure are little affected by experimental parameters such as the system pressure (28). Thus, the reverse micellar core in the microemulsion used in this study (Wo = 10) should have an average diameter close to 4 nm. Since there is evidence that the average size of the nanoparticles produced via RESOLV is dependent on the size of the pre-expansion reverse micelles in CO2 (9), it may be more than just a coincidence that the metal sulfide nanoparticles are of average sizes comparable to that of the pre-expansion water core (Table 1). The... 

Water-in-C02 microemulsions with diameters in the order of several nanometers are prepared by a mixture of AOT and a Pn E-P04 co-surfactant. The CO2 microemulsions allow metal species to be dispersed in the nonpolar supercritical CO2 phase. By chemical reduction, metal ions dissolved in the water core of the microemulsion can be reduced to the elemental state forming nanoparticles with narrow size distribution. The palladium and rhodium nanoparticles produced by hydrogen reduction of Pd and Rh ions dissolved in the water core are very effective catalysts for hydrogenation of olefins and arenes in supercritical CO2. 

Similar transformations were used to demonstrate the possibility of using water/ CO2 microemulsions as reaction media, which were stabilized using the anionic perfluoropolyether ammonium carboxylate surfactant [PFPECOO] [NH4] [25]. No additional phase transfer catalysts are necessary under these conditions. For example, the reaction between potassium bromide and benzyl chloride to form benzyl bromide [Eqs. (2)-(4)] resulted in a much better yield in the H2O/CO2 system than in a conventional water-in-oil microemulsion. 

The use of water-in-C02 microemulsions in particle synthesis has been extended recently to palladium [425]. An aqueous solution of PdCl2 was dispersed in supercritical CO2, and the surfactants NaAOT and (about twice its concentration of) perfluoropolyether phosphate (PFPE-PO4) were used for microemulsion stabilization. As in other similar cases, the formation of microemulsion and particle synthesis took place within a high pressure cell (at 200 atm). Reduction of palladium was caused by hydrogen gas (unlike in other similar investigations) because of its... 

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