Microemulsion reverse

Reverse microemulsion, a rather soft method, has been used to prepare hollow titania microspheres [51] TiCLi was dissolved in HCl-acidified water and added to a solution of TX-lOO in cycloheptane after addition of ammonia and stirring 

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The issue of water in reverse micellar cores is important because water swollen reverse micelles (reverse microemulsions) provide means for carrying almost any water-soluble component into a predominantly oil-continuous solution (see discussions of microemulsions and micellar catalysis below). In tire absence of water it appears tliat premicellar aggregates (pairs, trimers etc.) are commonly found in surfactant-in-oil solutions [47]. Critical micelle concentrations do exist (witli some exceptions). 

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

Microstructure Effects on Transport in Reverse Microemulsions John Texter... 

Percolation in microemulsions and concomitant microstructural changes are the focal points of this review. A complete understanding of percolation phenomena in reverse microemulsions will require an understanding of droplet interactions and the associated thermodynamics of droplet fusion, fission, aggregation to form clusters of varying fractal... 

Electrical conductivity is an easily measured transport property, and percolation in electrical conductivity appears a sensitive probe for characterizing microstructural transformations. A variety of field (intensive) variables have been found to drive percolation in reverse microemulsions. Disperse phase volume fraction has been often reported as a driver of percolation in electrical conductivity in such microemulsions [17-20]. 

Lagues et al. [17] found that the percolation theory for hard spheres could be used to describe dramatic increases in electrical conductivity in reverse microemulsions as the volume fraction of water was increased. They also showed how certain scaling theoretical tools were applicable to the analysis of such percolation phenomena. Cazabat et al. [18] also examined percolation in reverse microemulsions with increasing disperse phase volume fraction. They reasoned the percolation came about as a result of formation of clusters of reverse microemulsion droplets. They envisioned increased transport as arising from a transformation of linear droplet clusters to tubular microstructures, to form wormlike reverse microemulsion tubules. 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

FIG. 7 Order parameter for disperse pseudophase water derived from self-diffusion data for water, decane, and AOT reverse microemulsion illustrated in Fig. 6. The Op and arrow denote the approximate onset of percolation in low-frequency conductivity and a breakpoint in water self-diffusion increase. The arrow labeled AOT shows a second continuous transition corresponding to the onset of AOT self-diffusion increase. 

FIG. 8 Low-frequency conductivity (a) of water, toluene, and AOT reverse microemulsions at 25°C as a function of acrylamide (cosurfactant) concentration, (wt%). The Op and arrow at f = 1.2% shows the approximate onset of percolation in low-frequency conductivity. 

FIG. 9 Measured self-diffusion coefficients at 25°C for toluene (A), water ( ), acrylamide ( , and AOT ( ) in water, toluene, and AOT reverse microemulsions as a function of cosurfactant (acrylamide) concentration, f (wt%). The breakpoint at about 1.2% acrylamide approximately denotes, the onset of percolation in electrical conductivity. 

Kim D, Oh S, Lee J (1999) Preparation of ultrafine monodispersed Indium-Tin Oxide particles in AOT-Based reverse microemulsions as nanoreactors. Langmuir 15(5) 1599-1603... 

Zarur AJ, Ying JY (2000) Reverse microemulsion synthesis of nanostructured complex oxides for catalytic combustion. Nature 403 65-67... 

Liu, J., Han, B., Li, G., Zhang, X., He, J. and Liu, Z. (2001) Investigation of nonionic surfactant Dynol-604 based reverse microemulsions formed in supercritical carbon dioxide. Langmuir,... 

Y203 Eu nanoparticles for potential use in FEDs have been prepared in nonionic reverse microemulsions.124 The particles were synthesized by the reaction between aqueous yttrium nitrate, europium nitrate, and ammonium hydroxide, by bulk precipitation in the reverse microemulsion... 

Walsh, D., Hopwood, J.D. and Mann, S. (1994) Crystal tectonics construction of reticulated calcium-phosphate frameworks in bicontinuous reverse microemulsions. Science, 264, 1576-1578. 

Arriagada, F.J., and Osseo-Asare, K. (1995) Synthesis of nanosize silica in aerosol OT reverse microemulsions./. Colloid Interface Sci. 170, 8-17. 

Generally, two common methods, the Stober method and the reverse microemulsion method are used for synthesis of silica nanoparticles. As derivatives of a sol-gel process, both methods involve hydrolysis of a silicon alkoxide precursor to form a hydroxysilicate followed by polycondensation of the hydroxysilicate to form a silica nanoparticle [44]. 

The reverse microemulsion method can be used to manipulate the size of silica nanoparticles [25]. It was found that the concentration of alkoxide (TEOS) slightly affects the size of silica nanoparticles. The majority of excess TEOS remained unhydrolyzed, and did not participate in the polycondensation. The amount of basic catalyst, ammonia, is an important factor for controlling the size of nanoparticles. When the concentration of ammonium hydroxide increased from 0.5 (wt%) to 2.0%, the size of silica nanoparticles decreased from 82 to 50 nm. Most importantly, in a reverse microemulsion, the formation of silica nanoparticles is limited by the size of micelles. The sizes of micelles are related to the water to surfactant molar ratio. Therefore, this ratio plays an important role for manipulation of the size of nanoparticles. In a Triton X-100/n-hexanol/cyclohexane/water microemulsion, the sizes of obtained silica nanoparticles increased from 69 to 178 nm, as the water to Triton X-100 molar ratio decreased from 15 to 5. The cosurfactant, n-hexanol, slightly influences the curvature of the radius of the water droplets in the micelles, and the molar ratio of the cosurfactant to surfactant faintly affects the size of nanoparticles as well. 

In a reverse microemulsion, the hydrolysis and polymerization of the silicate precursor occur in the water droplet, therefore, to dope dyes in the silica nanoparticles they must be water soluble. However, a number of organic dye molecules are hydrophobic, requiring modifications prior to doping. Several methods are available to link a hydrophobic dye molecule to a water soluble group. A simple and effective example is to link a hydrophilic dextran to the dye molecules [8]. This modification can greatly enhance the water solubility of hydrophobic dye molecules, but will increase the cost of resultant DDSNs. 

In summary, a suitable association between dye molecules and the silica matrix is necessary for synthesis of DDSNs. Without the presence of chemical bonds or electronic interactions, the dye molecules will leak out from silica nanoparticles through the silica pores [22], Such DDSNs will provide unstable florescence signals and cannot be used as a labeling agent in bioanalysis. Meanwhile, water solubility is critical for a dye molecule when using a reverse microemulsion method to make the DDSNs. 

H. Althues, and S. Kaskel, Sulfated zirconia nanoparticles synthesized in reverse microemulsions preparation and catalytic properties, Langmuir 18, 7428—7435 (2002). 

It is well known that the aqueous phase behavior of surfactants is influenced by, for example, the presence of short-chain alcohols [66,78]. These co-surfactants increase the effective value of the packing parameter [67,79] due to a decrease in the area per head group and therefore favor the formation of structures with a lower curvature. It was found that organic dyes such as thymol blue, dimidiiunbromide and methyl orange that are not soluble in pure supercritical CO2, could be conveniently solubihzed in AOT water-in-C02 reverse microemulsions with 2,2,3,3,4,4,5,5-octafluoro-l-pentanol as a co-surfactant [80]. In a recent report [81] the solubilization capacity of water in a Tx-lOO/cyclohexane/water system was foimd to be influenced by the compressed gases, which worked as a co-surfactant. 

Zhao XJ, Bagwe RP, Tan WH (2004) Development of organic-dye-doped silica nanoparticles in a reverse microemulsion. Adv Mater 16 173... 

Arriagada FJ, Osseoasare K (1992) Phase and Dispersion Stability Effects in the Synthesis of Silica Nanoparticles in a Nonionic Reverse Microemulsion. Colloids Surf 69 105-115... 

Bagwe RP, Yang CY, Hilliard LR, Tan WH (2004) Optimization of dye-doped silica nanoparticles prepared using a reverse microemulsion method. Langmuir 20 8336-8342... 

Arriagada FJ, Osseoasare K (1994) Silica Nanoparticles Produced in Aerosol Ot Reverse Microemulsions - Effect of Benzyl Alcohol on Particle-Size and Polydispersity. J Dispers Sci Technol 15 59-71... 

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