AOT-based microemulsions

Under comparable conditions, the reaction rates of the octyl decanoate synthesis by chromobacterium viscosum lipase in AOT-based microemulsions or immobihzed in AOT-based organogels were similar [286,287],... 

Marchand KE, Tarret M, Lechaire JP, Normand L, Kasztelan S, Cseri T (2003) Investigation of AOT-based microemulsions for the controlled synthesis of MoSx nanoparticles an electron microscopy study. Colloids Surf A 214 239-248... 

In contrast to nonionic surfactants, ionic surfactants build up a high zeta-po-tential at the water-oil interface which can also can influence the enzyme activity. Most investigated systems used AOT as the surfactant because its phase behaviour is well understood. However, AOT is often not very suitable, because it can totally inhibit enzymes (e.g. the formate dehydrogenase from Candida bodinii). The usage of lipases in AOT-based microemulsions is generally unfavourable as AOT is an ester that is hydrolysed itself. 

Surprisingly, esterification of fatty acids with simple sugars, such as glucose and mannitol, in AOT-based microemulsions did not take place at all [82]. No reaction was seen with either of two different lipases. This is probably due to poor phase contact between the very hydrophilic sugar molecule in the water pool and the fatty acid that resides in the hydrocarbon domain. Sugar monoesters can be produced in high yields by lipase-catalyzed esterification in a water-free medium [90]. 

A similar approach for ILs based on an amphiphilic anion and imidazolium cation, [C mim][OT], OT being the l,4-bis(2-ethylhexylsulfosuccinate) anion, has been proposed. The use of sodium l,4-bis(2-ethylhexyl) sulfosuccinate, AOT, in the formulation of water-in-oil microemulsions is well reported in the literature [106-108]. The incorporation of IL to the formulation of microemulsions has been studied from different points of view (i) AOT-based microemulsions incorporating... 

E. Fernandez, L. Garcfa-Rfo, J.R. Leis, J.C. Mejnto, and M. Perez-Lorenzo, Michael addition and ester aminolysis in w/o AOT-based microemulsions, 2005, New J. Chem. 29, 1594-1600. 

Structural and Spectral Characteristics of Various Lipases in AOT-Based Microemulsions... 

In addition to AOT-based microemulsions, a nonionic reverse micelle (GGDE/TX-100), containing a functional nonionic surfactant N-gluconyl glutamic acid didecyl ester (GGDE) and Triton X-100 (TX-lOO), was evaluated as a good alternative in which YADH exhibits higher catalytic activity and stability [89]. 

By adding 1-alkanols to AOT-based w/o microemulsions, some proteins (ribonucle-ase, lysozyme, alpha-chymotrypsin, pepsin, bovine serum albumin, and catalase) are readily expelled, while the major part of the surfactant remained in solution [171]. 

Time-resolved luminescence quenching measurements using the probe Tb(pyridine-2,6-dicarboxylic acid)i and the quencher bromophenol blue show the existence of micellar clusters in AOT-based w/o microemulsions. The fast exchange appearing over several microseconds was attributed to intracluster quenching, whereas the slow exchange on the millisecond time scale was attributed to intercluster exchange [243]. 

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... 

Microemulsions are dynamic systems in which droplets continually collide, coalesce, and reform in the nanosecond to millisecond time scale. These droplet interactions result in a continuous exchange of solubilizates. The composition of the microemulsion phase determines the exchange rate through its effect on the elasticity of the surfactant film surrounding the aqueous microdomains. Compared with nonionic surfactant-based microemulsions, AOT reverse micelles have a more rigid... 

Using a variation on the microemulsion-mediated process, catalytic bi-metalhc Ru-Cu oxides supported by sihca were made through the controlled hydrolysis/polymerization of sodiiun metasihcate, copper nitrate and rutheniiun chloride via a single microemulsion protocol [ 135]. With AOT and SDS-based microemulsion, a high specific surface area (-400 m /g) and imiform pore size... 

Figure 2.3 Intermediate scattering functions obtained for an AOT-based o/w-droplet microemulsion using NSE. The solid lines are fits with a single exponential function yielding an effective diffusion coefficient. Note that this was the first NSE study of a microemulsion showing the calculation of k on the basis of the intermediate scattering functions. (From Ref. [45], reprinted with permission of the American Physical Society.)...

The alkaline fading of crystal violet (4-Me2NC6H4)3C+ in cationic W/O microemulsions of CTABr-alcohol-cyclohexane occurs in the water pools [113]. The corresponding reactions of crystal violet and malachite green, (4-Me2NC6H4)2C Ph, in anionic W/O AOT-isooctane microemulsions also occur in the water pools, and rate data were analyzed in terms of their size and the competing base-promoted hydrolysis of AOT [114]. This reaction with crystal violet is strongly inhibited by ClOj, which pairs with the carbocation and blocks nucleophilic attack,... 

NMR self-diffusion measurements indicated that all microemulsions consisted of closed water droplets and that the structure did not change much during the course of reaction. Hydrolysis was fast in microemulsions based on branched-chain anionic and nonionic surfactants but very slow when a branched cationic or a linear nonionic surfactant was employed (Fig. 11). The cationic surfactant was found to form aggregates with the enzyme. No such interactions were detected with the other surfactants. The straight-chain, but not the branched-chain, alcohol ethoxylate was a substrate for the enzyme. A slow rate of triglyceride hydrolysis for a Ci2E4-based microemulsion compared with formulations based on the anionic surfactant AOT [61,63] and the cationic surfactant cetyltrimethylammonium bromide (CTAB) [63] was observed in other cases also. Evidently, this type of lipase-catalyzed reaction should preferably be performed in a microemulsion based on an anionic or branched nonionic surfactant. Nonlipolytic enzymes such as cholesterol oxidase seem to function well in microemulsions based on straight-chain nonionic surfactants, however [64]. CTAB was reported to cause slow inactivation of different types of enzymes [62,64,65] and also, in the case of Chromobacterium viscosum lipase [66], to provide excellent stability. 

As displayed in Table 1, the highest incorporation of starch nanopartides within reverse micelles was found by using the AOT/isooctane microemulsion system. Based on these results, the AOT/isooctane system was used to prepare surfactant coated starch nanopartides. To isolate the AOT-coated starch nanopartides, the isooctane was removed under reduced pressure. Previous studies in our laboratory showed that toluene was a preferred organic medium in which to perform Novozyme 435 catalyzed transesterification reactions(29). Hence, the AOT-coated starch nanospheres were solubilized in toluene and evaluated for modification reactions. 

L. Garcfa-Rio, J.C. Mejnto, and M. Perez-Loienzo, Ester aminolysis by morpholine in AOT-based water-in-oil microemulsions, 2006, J. Colloid Interface Sci. 301,624-630. 

W/O microemulsions have also been shown to be an effective system for the lipase-catalyzed enantioselective esterifications of racemic substrates [115,117]. It has been reported that the reaction rate for the esterification of the (-)-enantiomer of menthol with fatty acids catalyzed by P. simplicissimum lipase in AOT-based systems is seven times faster than the esterification rate of the (H-)-enantiomer... 

Moreover, the enantioselective synthesis of ibuprofen esters catalyzed by Candida cylindracea lipase was also studied in a similar system [115]. The enanti-oselectivity displayed by the lipase was much higher in AOT/isooctane microemulsions than in isooctane. The use of lipases to prepare large quantities of an optically enriched product has also been demonstrated in CTAB-based microemulsions [89]. Resolution of ( )-menthol by C. cylindracea lipase-catalyzed esterification with propionic anhydride in a nonionic reverse micellar system in an ultraiiltration manbrane reactor has been recently described [117]. 

---