Water in oil microemulsion

O. A. El Seoud, Reversed micelles and water-in-oil microemulsions, in W. L. Hinze (ed.) Organized Assemblies in Chemical Analysis, Vol. 1, Wiley, New York, NY 1994, 1. 

Hernandez J, Solla-Gullon J, Herrero E. 2004. Gold nanoparticles synthesized in a water-in-oil microemulsion Electrochemical characterization and effect of the surface structure on the oxygen reduction reaction. J Electroanal Chem 574 185-196. 

Water-in-Oil Microemulsions Protein Encapsulation and Release Douglas G. Hayes... 

Water-in-oil microemulsions (w/o-MEs), also known as reverse micelles, provide what appears to be a very unique and well-suited medium for solubilizing proteins, amino acids, and other biological molecules in a nonpolar medium. The medium consists of small aqueous-polar nanodroplets dispersed in an apolar bulk phase by surfactants (Fig. 1). Moreover, the droplet size is on the same order of magnitude as the encapsulated enzyme molecules. Typically, the medium is quite dynamic, with droplets spontaneously coalescing, exchanging materials, and reforming on the order of microseconds. Such small droplets yield a large amount of interfacial area. For many surfactants, the size of the dispersed aqueous nanodroplets is directly proportional to the water-surfactant mole ratio, also known as w. Several reviews have been written which provide more detailed discussion of the physical properties of microemulsions [1-3]. 

The reductive elimination of vicinal dihalides has been accomplished by using many reagents, including the use of aqueous media.16 An interesting method is the reductive elimination of vicinal dihalides by an electrochemical method using vitamin Bi2 in a water-in-oil microemulsion (Eq. 6.8).17... 

Surfactant molecules can be considered as building blocks for certain forms of geometry in colloidal chemistry. Various forms of association molecules can be obtained as the concentration of surfactant in water is increased and/or physicochemical conditions are changed (e.g. CMC, Craft-point, etc.). Figure 2 schematically shows the most likely structural configurations and assemblages of surfactants association in an aqueous system (26). Upon addition of oil and a short-chain alcohol, for example, one can convert the oil-in-water micelles into water-in-oil microemulsions. It is therefore possible to induce a transition from one structure to another by changing the physicochemical conditions such as temperature, pH and addition of mono or di-valent cations to the surfactant solution. It should be also noted that the sur-... 

Zhi, J., Wang, Y., Lu, Y., Ma, J. and Luo, G. (2006) In situ preparation of magnetic chitosan/Fe304 composite nanoparticles in tiny pools of water-in-oil microemulsion. Reactive and Functional Polymers, 66, 1552-1558. 

Fig. I Schematic diagram of water-in-oil microemulsion for synthesis of silica nanoparticles...

Fig. 2 TEM images of variable sizes of silica NPs obtained from a quaternary water-in-oil microemulsion using different organic solvents (a) cyclohexane, (b) n-pentane, (c) n-hexane, (d) n-heptane, (e) n-decane, and (f) zt-hexadecane. Reproduced with permission from Ref. [56]...

Usually, activities of enzymes (hydrogenases included) are investigated in solutions with water as the solvent. However, enhancement of enzyme activity is sometimes described for non-aqueous or water-limiting surroundings, particular for hydrophobic (or oily) substrates. Ternary phase systems such as water-in-oil microemulsions are useful tools for investigations in this field. Microemulsions are prepared by dispersion of small amounts of water and surfactant in organic solvents. In these systems, small droplets of water (l-50nm in diameter) are surrounded by a monolayer of surfactant molecules (Fig. 9.15). The water pool inside the so-called reverse micelle represents a combination of properties of aqueous and non-aqueous environments. Enzymes entrapped inside reverse micelles depend in their catalytic activity on the size of the micelle, i.e. the water content of the system (at constant surfactant concentrations). 

Similar investigations have been carried out on water in oil microemulsions. A microemulsion is a clear, transparent, and stable system consisting of essentially monodisperse oil in water (OAV) or water in oU (W/O) droplets with diameters generally in the range of 10-200 nm. Microemulsions are transparent because of their small particle size, they are spherical aggregates of oil or water dispersed in the other liquid, and they are stabilized by an interfacial film of one or more surfactants. 

The packing ratio also explains the nature of microemulsion formed by using nonionic surfactants. If v/a 1 increases with increase of temperature (as a result of reduction of a ), one would expect the solubilisation of hydrocarbons in nonionic surfactact to increase with temperature as observed, until v/a l reaches the value of 1 where phase inversion would be expected. At higher temperatures, va l > 1 and water in oil microemulsions would be expected and the solubilisation of water would decrease as the temperature rises again as expected. 

Oil rich phase Oil rich phase Water in oil microemulsion... 

RD.I. Eletcher and D.I. Horsup Droplet Dynamics in Water-in-Oil Microemulsions and Macroemulsions Stabilized by Non-Ionic Surfactants. J. Chem. Soc. Earaday Trans. I 88, 855 (1992). 

Pto.45Pdo.45Bio.n/C in 0.2 M NaOH + 0.1 M EC, at various potentials (each 100 mV) from 130 to 1030 mV/RHE (1) and chromatograms of the anodic outlet of a SAMEC working with a Pto.45 Pdo.45Bio.i/C anode without fuel recirculation (2). Electrocatalysts were prepared according to the water-in-oil microemulsion method. 

Figure1.22 Polarization curvesfortheoxidation ofO.Ol M NaBH4 in a 1 M NaOH solution recorded on (1) Pt (50 wt%)/C and (2) Au (50wt%)/C i = 20mVs Q.= 1000rpm, T=20°C. Catalysts were prepared according to the water in oil microemulsion method.

Demarconnay et al. used a method derived from that of the so-called water-in-oil microemulsion method to prepare well dispersed Ag/C catalysts [116]. The onset of the oxygen reduction wave is only shifted by 50 mV towards lower potentials on an Ag/C catalyst compared with that obtained on a Pt/C catalyst and the limiting current... 

Figure 1.23 j(E) polarization curves recorded at a rotation rate 2 = 2500 rpm in an 02-saturated 0.1 M NaOH electrolyte (T=20°C, v = 5 mVs ) for (dashed line) 20wt%Ag/C and (solid line) 20wt% Pt/C. Ag/C catalyst was prepared according to the water-in-oil microemulsion method, whereas Pt/C catalyst was prepared using the Bonnemann method. 

In the water-flooding process, mixed emulsifiers are used. Soluble oils are used in various oil-well-treating processes, such as the treatment of water injection wells to improve water injectivity and to remove water blockage in producing wells. The same method is useful in different cleaning processes with oil wells. This is known to be effective since water-in-oil microemulsions are found in these mixtures, and with high viscosity. The micellar solution is composed essentially of hydrocarbon, aqueous phase, and surfactant sufficient to impart micellar solution characteristics to the emulsion. The hydrocarbon is crude oil or gasoline. Surfactants are alkyl aryl... 

Yener DO, Giesche H (2001) Synthesis or pure and manganese-, nickel- and zinc-doped ferrite particles in water-in-oil microemulsions. J Am Ceram Soc 84 1987-1995... 

Gold nanoparticles from 2.5 to 5 nm sizes have also been prepared by using a biphasic Winsor II [126] (a water-in-oil microemulsion that is in equilibrium with the excess water phase) type microemulsion of diethyl ether/AOT/water. The surfactant, AOT, performs the dual role of forming a microemulsion and the transferring of charged metal ions from the aqueous to organic phase. This provides gold nanoparticles, which are readily dispersed in the nonpolar phase. 

Pillai V, Kumar P, Hou M J, Ayyub P, Shah DO (1995) Preparation of Nanoparticles of Silver-Halides, Superconductors and Magnetic-Materials Using Water-in-Oil Microemulsions as Nano-Reactors. Adv Colloid Interface Sci 55 241-269... 

Fletcher PDl, Howe AM, Robinson BH (1987) The Kinetics of Solubilisate Exchange between Water Droplets of a Water-in-Oil Microemulsion. J Chem Soc Faraday Trans 1 83 985-1006... 

Chang CL, Fogler HS (1997) Controlled formation of silica particles from tetraethyl orthosUicate in nonionic water-in-oil microemulsions. Langmuir 13 3295-3307... 

Arriagada FJ, Osseo-Asare K (1999) Synthesis of nanosize silica in a nonionic water-in-oil microemulsion Effects of the water/surfactant molar ratio and ammonia concentration. J Colloid Interface Sci 211 210-220... 

Ma ZN, Friberg SE, Neogi P (1988) Observation of Temporary Liquid-Crystals in Water-in-Oil Microemulsion Systems. Colloids Sirnf 33 249-258... 

Tata M, Banerjee S, John VT, Waguespack Y, McPherson GL (1997) Fluorescence quenching of CdS nanocrystallites in AOT water-in-oil microemulsions. Colloids Surf A 127 39-46... 

Agostiano A, Catalano M, Curri ML, Della Monica M, Manna L, Vasanelli L (2000) Synthesis and structural characterisation of CdS nanoparticles prepared in a four-components water-in-oil microemulsion. Micron 31 253-258... 

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviom" and the corresponding activity of enzymes the application of these media can lead to favomable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes. 

Fletcher PDl, Parrot D (1989) Protein-partitioning between water-in-oil microemulsions and conjugate aqueous phases. In Pileni MP (ed) Structure and reactivity in reverse micelles. Elsevier, Amsterdam, p 303... 

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