Microemulsion stability, fluorinated

Holmes et al. (1998) performed two enzymatic reactions, the lipase-catalyzed hydrolysis of y>-nitrophenol butyrate and lipoxygenase-catalyzed peroxidation of linoleic acid, in w/c microemulsions stabilized by a fluorinated two-chained sulfosuccinate surfactant (di-HCF4). The activity of both enzymes in the w/c microemulsion environment was found to be essentially equivalent to that in a water/heptane microemulsion stabilized by Aerosol OT, a surfactant with the same headgroup as di-HCF4. The buffer 2-(A-morpholino)ethanesulfonic acid (MES) was used to fix the pH in the range 5-6. 

Table 1. Aqueous Phase Critical Micelle Concentrations (erne s), Limiting Surface Tensions yeme s and Microemulsion Stability Pressures for Fluorinated Surfactants.

Fluorinated surfactants can also serve as miceUar stabilizers for nanoparticles in water-in-supercritical CO2 (SCCO2) microemulsions. Recently, Tsang described Ru nanoparticles as catalysts in the presence of ammonium perfluorotetradecano-... 

Bioconversion can also be conducted in water-in-C02 microemulsions. In one case, fluorinated two-chain sulfosuccinate surfactants have been used to stabilize microemulsions in which the enzyme was located within the dispersed water droplets [39]. Two reactions, a lipaseotalyzed hydrolysis ofp-nitrophenol butyrate and a lipoxygenase-catalyzed peroxidation of linoleic acid, were investigated [Eqs. (11) and (12)]. The pH was controlled by the buffer MES [2-(N-morpho-Hno)ethanesulfonic acid] in the range of 5-6. Both enzymes showed good activity in the water-in-C02 microemulsion, largely identical to that observed in water-inheptane microemulsions. 

Beside modification of the PT catalyst by incorporation of C02-philic groups, other approaches to the problem of the poor solubility of polar compounds in SCCO2 were envisaged. Fluorinated surfactants (perfluoro-polyether ammonium carboxylates) were used as stabilizers of H2O/SCCO2 microemulsions.The nucleo-... 

While the stabilization of W/C microemulsions with Krytox-type polymers or molecular surfactants has been largely reported [66, 67], examples implying amphiphilic copolymers are scarce in literature. These examples—described below— are exclusively based on fluorinated poly((meth)acrylates) as a consequence of their lower surface tension compared to hydrocarbons and poly(siloxanes). 

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. 

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