Carbon dioxide microemulsions with water

Ryoo, W., Webber, S.E. and Johnston, K.P. (2003) Water-in-carbon dioxide microemulsions with methylated branched hydrocarbon surfactants. Industrial and Engineering Chemistry Research, 42 (25), 6348-6358. 

Flarrison K, Goveas J, Johnston KP, O Rear EA. Water-in-carbon dioxide microemulsions with a fluorocarbon-hydrocarbon hybrid surfactant. Langmuir... 

Lee CT, Psathas PA, Zielger KJ, Johnston KP, Daib HJ, Cochran HD, Melnichenko YB, Wignall GD. Formation of water-in-carbon dioxide microemulsions with a cationic surfactant a small-angle neutron scattering study. J Phys Chem B 2000 104 11094-11102. 

Liu Z, Erkey C. Water in carbon dioxide microemulsions with fluorinated analogues of AOT. Langmuir 2001 17 274-277. 

Liu, Z. Erkey, C. Water in Carbon Dioxide Microemulsions with Fluorinated Analogues of AOT. Langmuir 2001, 17(2), 274-277. 

Micro emulsions can be formulated with carbon dioxide in supercritical state instead of a hydrocarbon as nonaqueous solvent. Fluorinated surfactants are commonly used to prepare such microemulsions. Water-in-carbon dioxide microemulsions can be made and the droplet size has been found to be similar to the size of the droplets of water-in-hydrocarbon micro emulsions with similar composition [21]. Such a microemulsion was used for conversion of benzyl chloride to benzyl bromide using KBr as hydrophilic nucleophile. The yield was an order of magnitude higher in the carbon dioxide microemulsion than in a conventional microemulsion at similar conditions, a fact that has been ascribed to low interfacial viscosity [22]. The big advantage with these micro emulsions is the environmental friendliness and the ease of work-up associated with carbon dioxide as solvent. 

Nanocrystalline metal (silver and copper) and metal sulfide (silver sulfide, cadmium sulfide, and lead sulfide) particles were prepared via RESOLV (Rapid Expansion of a Supercritical Solution into a Liquid SOLVent) with water-in-carbon dioxide microemulsion as solvent for the rapid expansion. The nanoparticles were characterized using UV/vis absorption. X-ray powder diffraction, and transmission electron microscopy methods. The results of the different nanoparticles are compared and discussed in reference to those of the same nanoparticles produced via RESOLV with the use of conventional supercritical solvents. 

The uses of fluorosurfactants to stabilize water-in-carbon dioxide (w/c) microemulsions are reviewed. A systematic study with fluorosuccinate surfactants of the effects of the extent of fluorination of hydrophobic chains on the stability and structures of these w/c phases is described. Therefore, it has been possible to delineate a structure-function relationship for these fluorosurfactants with reference to their efficiency of water-in-carbon dioxide microemulsion formation. An important finding is that one of these surfactants, namely sodium bis(l//,l//-perfluoropentyl)-2-sulfosuccinate (di-CF4), is able to stabilize w/c microemulsions close to the bottle pressure of a normal CO2 cylinder. Such efficient surfactants of this kind have obvious advantages for potential practical applications of CO2. 

A range of fluorinated anionic surfactants, which are structural relatives of Aerosol-OT, were synthesized and characterized, with the aim of investigating the effects of chemistry on the structure and stability of water-in-carbon dioxide microemulsions. The dilute aqueous phase behavior was studied to check for chemical purity and fuUy characterize the compounds. Once appropriate measures were taken to achieve sufficient purity, the surface excesses measured by both tensiometry and neutron reflection measurements agreed well, and the surface tensions were consistent with a prefactor of 2 in the Gibbs equation. 

When this pressure drops, it can be built-up again by water flooding. Unfortunately, after these primary and secondary processes, there still remains up to 70% of the oil adsorbed on the porous clays. Consequently, in recent years, there have been tremendous efforts made to develop tertiary oil recovery processes, namely carbon dioxide injection, steam flooding, surfactant flooding and the use of microemulsions. In this latter technique, illustrated in Fig. 1, the aim is to dissolve the oil into the microemulsion, then to displace this slug with a polymer solution, used for mobility control, and finally to recover the oil by water injection ( 1). 

The common surfactants discussed repeatedly here and elsewhere are in general unsuitable for water-supercritical carbon dioxide emulsions of all types (including microemulsions). Selections are made, to start with, on the basis of the water/ carbon dioxide solubility behavior. A surfactant used in some of the initial studies is ammonium carboxylate perfluoropolyether [24,25]. Subsequently, a variety of triblock co-polymers, e.g. poly(propylene oxide-b-ethylene oxide-b-propylene... 

Metallic particles can be prepared by reducing salts dissolved in the droplets. For example, copper particles can be produced using hydrazine as a reducing agent (Pileni, 1993). Composite particles with a core of one metal surrounded by another have also been reported (see Belloni, 1996). Preparation of nanoparticles of various metals has been reviewed by Capek (2004). In some cases, supercritical carbon dioxide (COj) has been used instead of a hydrocarbon as the continuous phase in the microemulsion, which provides a system where the phase behavior can be relatively easily controlled by changing pressure and eliminates the use of volatile solvents. There is considerable interest in making nanoscale particles of semiconductors such as cadmium sulfide (CdS) using water-in-oil microemulsions because their electronic properties are different from those of bulk crystals (see Pileni, 1993). 

Tsang SC, Zhu J, Yu KMK. A new oxidation catalyst system using fluorous cobalt(ll) species in water-supercritical carbon dioxide (C-H free environment). Catal Lett 2006 106 123-6. Martin CA, McCrann PM, Angelos GH, Jaeger DA. Reactions in microemulsion media nucleophilic displacement reaction of benzyl chloride with bromide ion. Tetrahedron Lett... 

More recently, titanium dioxide particles have been produced using supercritical CO2 (77). In this case, carbon dioxide is used as the organic phase of the microemulsion. Two containers were placed inside a pressure vessel with one container partially filled with an alkoxide precursor and the other with a solution of water and fluorinated surfactants. The vessel was then pressurized with carbon dioxide and heated until supercritical conditions were attained. The alkoxide is soluble in carbon dioxide, and the aqueous surfactant solution forms a stable microemulsion with the supercritical carbon dioxide. Over time, the alkoxides mix with the micelles and react with the water to form titanium dioxide. The product characteristics depend significantly on the solubility of the alkoxides in supercritical carbon dioxide. 

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