Surfactants AOT

All chemicals are of reagent grade bidistilled water is used throughout this work. Anionic surfactant AOT is vacuum-dried for 24 h at 333 K directly before use. Water-free hydrocarbon (e.g., extra dry isooctane, water <30 ppm) is used for a ME preparation. All glassware is air-dried at 393 K. 

The reverse micelles stabilized by SDS retard the autoxidation of ethylbenzene [27]. It was proved that the SDS micelles catalyze hydroperoxide decomposition without the formation of free radicals. The introduction of cyclohexanol and cyclohexanone in the system decreases the rate of hydroperoxide decay (ethylbenzene, 363 K, [SDS] = 10 3mol L [cyclohexanol] =0.03 mol L-1, and [cyclohexanone] = 0.01 mol L 1 [27]). Such an effect proves that the decay of MePhCHOOH proceeds in the layer of polar molecules surrounding the micelle. The addition of alcohol or ketone lowers the hydroperoxide concentration in such a layer and, therefore, retards hydroperoxide decomposition. The surfactant AOT apparently creates such a layer around water moleculesthat is very thick and creates difficulties for the penetration of hydroperoxide molecules close to polar water. The phenomenology of micellar catalysis is close to that of heterogeneous catalysis and inhibition (see Chapters 10 and 20). 

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. 

The surfactants AOT (0.8 M) and Lecithin (0.4 M) were dissolved in isooctane. The aqueous phase, consisting of either a 10 M HAuCU solution, or alternate aliquots of 10 M HAuCU and NaBH4 (10 M), was then added to the surfactant-containing organic phase until the desired Wo (moles water/moles AOT) was reached. The reactions proceeded at room temperature. We carried out three sets of experiments to synthesize nanoparticles of different morphologies. 

The ultrafiltration of the microemulsion is a very useful operation for separating water and oil in these mixtures [117-120]. Because of the limited availability of solvent stable membranes, most of the work pubHshed so far was performed using ceramic membranes, which show a high adsorption of surfactant at the membrane surface and comparably low rejection rates of reverse micelles. Using electro ultrafiltration, where the concentration polarisation phenomenon of the reverse micelles (using the ionic surfactant AOT) at the membrane surface is depressed by asymmetric high voltage electrical fields, the rejection rates can be increased,but not to economical values [121,122]. 

Previous work has shown that binary surfactant systems containing Dowfax 8390 and the branched hydrophobic surfactant AOT can form Winsor III systems with both PCE and decane whereas DOWFAX 8390 by itself cannot (Wu et. al. 1999). This binary surfactant system was used in conjunction with hydrophobic octanoic acid to help with phase behavior and lessen the required concentration of CaCl2. Since this formulation is rather complicated, questions about field robustness arise. Thus, for the phase behavior studies presented here, we used the simple binary system of the nonionic TWEEN 80 and the branched hydrophobic AOT, and we optimized the NaCl concentration to give the Winsor Type III system. The lesser electrolyte concentration requirement for the binary TWEEN 80/ AOT system helps to decrease the potential for undesirable phase behavior such as surfactant precipitation, thereby increasing surfactant system robustness. 

Fig. 23. Top Characteristic frequency fc versus surfactant (AOT) concentration in cyclohexane, 22.0 °C. Curve through data points calculated according to43). Bottom Amplitude factors of the field effect measurements normalized with respect to the applied dc field of AOT/CgHij solutions Upper curve (positive amplitude, solid circles) Chemical excess losses. Lower curve (negative amplitudes, open circles) orientational field effect [Ber. Bunsenges. Phys. Chem. 79, 667 (1975)]...

Since most of the studies concerning protein extraction with reverse micelles have used a representative surfactant, AOT and trioctyl methyl ammonium chloride (TOMAC), to form reverse micelles, the protein extraction studies using a novel surfactant shed light on the important factors in the design of reverse micelles suitable for protein extraction. In particular, a comparative investigation concerning protein extraction efficiency with a variety of synthetic surfactants clarified the role of the hydrophobic tails of the surfactant on protein extraction [9]. 

The surfactant AOT forms reverse micelles in non-polar fluids without addition of a cosurfactant, and thus it is possible to study simple, water/AOT/oil, three component systems. To determine micelle structure and behavior in water/AOT/oil systems, investigators have studied a wide range of properties including conductivity (15), light (JL ), and neutron (12) scattering, as well as solution phase behavior (1 ). From information of this type one can begin to build both microscopic models and thermodynamic... 

Most of the early work involving microemulsions in supercritical fluids utilized the supercritical alkanes, ethane and propane, with the surfactant AOT. Table 1 gives a summary of the surfactant systems that have been studied in supercritical hydrocarbon solvents. More recently, there has been some success with the formation of... 

Anionic Surfactant AOT has been studied extensively since it forms reverse micelles readily in a variety of organic solvents, even without a co-surfactantf28 >. Figure 6 shows the solvatochromic shifts of 0.0002 M pyridine-N-oxide in solutions of AOT in SCF ethane at 345 bar. No water was added to the system, but it is likely that Wo was 1 given the difficulty of completely dehydrating AOT(22). The pressure was fixed at 345 bar so that all of the AOT solutions would be in the one-phase region(16). Notice how closely the results for ethane match those for... 

The surfactant AOT ( purum grade, Fluka) was purified as described by Kotlarchyk 22). The AOT solution was filtered through a 0.2-)im Millipore filter prior to drying in vacuo for eight hours. The AOT was stored in a desiccator over anhydrous calcium sulfate. The molar water-to-AOT ratio (W) was assumed to be 1 in the purified, dried solid (2J ). Water was distilled and filtered through a Millipore Milli-Q system. Ethane, propane ("CP" grade, Linde), and xenon (Research grade, Linde) were used as received. The alkanes had a reported purity of >99% (Aldrich) and were used as received. 

The reverse microemulsion phase chosen was a well documented one it included AOT as the main surfactant. AOT or Bis(2-ethylhexyl)sulfosuccinate is an edge-shape ion, as represented in Fig. 1. It favors the formation of spherical water-in-oil droplets at low water content but, on increasing the water-to-surfactant molar ratio w, interconnected cylinders have been reported [7, 3]. The choice of AOT was made in order to concentrate the reaction medium, i.e. increase the volume of dispersed phase and so the yield of the formation of MoSx with respect to the total microemulsion volume. 

Fig. 6. Schematic representation of an interface with a surfactant (AOT), a cosurfactant (NP-5) and organic molecules...

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. 

As already pointed out the first work directly measuring the deformation dynamics in an o/w-droplet microemulsion using NSE was published by Huang et al. [45]. In this work, a microemulsion based on the surfactant AOT was studied and it was shown that the intermediate scattering functions contain information about the centre of mass diffusion and in addition also contributions from the deformation dynamics. The intermediate scattering functions obtained in this work are shown in Fig. 2.3. 

Coordination of the -SOT group of the ligand to the rhodium may, indeed, be important in the observed effect. It was found by Buriak and Osborn [146,147] that in microemulsions, prepared with the surfactant AOT (Scheme 3.11) the sulfonate group of AOT did coordinate to rhodium in the [Rh (-)-DBPP (NBD)]+ complex. It was suggested that this led to an easier deprotonation of an intermediate dihydride species in case of RS03 than for example in case of T (Scheme 3.28), i.e. to a switch from a dihydride route of hydrogenation to a monohydride pathway. How this would lead to high enantioselection still remains elusive. 

The presence of AOT resulted in an about twofold increase in initial rate of conversion in toluene, whereas T 20 did not improve activity. A similar effect was also observed with PEG-modified SC (PEG-SC). The PEG attachment by itself did not inerease the rate of reaction significantly, although a larger enhancement was observed with the surfactants. In the case of T 20, PEG modification resulted in a twofold increase in activity, not observed with the native enzyme. With hexane as solvent, the PEG modification resulted in a slight increase in activity compared to the activity of the native enzyme. However, given the 23% estimated error (Table 1) in determination of the rate, this difference is not significant. Whereas the surfactant AOT appeared to negatively affect the activity in hexane for the native enzyme, the PEG modification appeared to reduce this deleterious effect. 

In eq 1 vd2o is the volume of a water molecule. Hence, can be estinmted as 115 5 and the intercept is consistent with a radius for die polar core of a dry micelle of around 11 A. This head group area Afj can also be calculated from high Q SANS data with the Porod equation, as described elsewhere (13, 86). These results indicate that fluoro-surfactants at the water-COa interface adopt a lower packing density than a related hydrocarbon surfactant (AOT) at analogous water-alkane interfaces (86). 

Figure 4. (a) Surfactants AOT (AerosoUOT) Sodium bis (2-ethyl-l-hexyl)sulfosuccinate, surfactant A (sodium bis (2,4,4-trimethyl-1-pentyl)sulfosuccinate) and surfactant B (sodium bis (3,5,5-trimethyl-1-hexyl) sulfosuccinate. (b) SANS data obtained after subtracting the cell sc-C02 background for surfactant K as a function of concentration at 0.15 ( ) and 0.10 (O) mol dm . T - 32X1 and 500 bar. The fits are to a polydisperse sphere model with = 14 1A and = 0.20 (footnote 37). (c) UV-vis spectrum of dimidium bromide dispersed in SC-CO2 with reversed micelles of surfactant B at 40X1 and 500 bar. The surfactant concentration is 0.025 mol dm. Reproduced with permission from Journal of the American Society, 2001, 123, 988-989. Copyright 2001 Am. Chem. Soc. 

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