CTAB-based microemulsion

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]. 

Rabe, C. Koetz, J. (2010). CTAB-based microemulsions with ionic liquids. Colloids and Surfaces A Physicochemical and Engineering Aspects, 354(1-3), 261-267. 

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. 

ILs can be incorporated in the microemulsion formulation as substituents of the polar and nonpolar phases or as surfactants. In recent studies, Koetz and coworkers [103] showed that the role of cosurfactant can also be assumed by an IL in the stabilization of water-in-oil microemulsions. SDS- and CTAB-based water-in-toluene/ pentanol microemulsions have been formulated with the aid of ethyl-methylimidazolium hexylsulfate, [C2mim][CgSOJ. Their experimental results showed that replacing water by the IL increases the isotropic phase region of the system. The authors assume the formation of a palisade layer (Scheme 13.4), where the IL plays a similar role like a cosmfactant, changes the spontaneous curvature of the interfacial film, and decreases the droplet size. 

To obtain a true k in MEEKC, it is important to trace the migration of the pseudostationary phase accurately. Sudan III, timepidium bromide, and quine, which have generally been used as tracers for micelles in MEKC, could not be employed as tracers for microemulsions consisting of sodium dodecylsulfate salt (SDS) or cetyltrimethylammonium bromide (CTAB), n-butanol and heptane (12). An iteration method based on a linear relationship between log k and the carbon number for alkylbenzenes (13) seems to provide a reasonable value of the migration time of the microemulsions. Dodecylbenzene shows a migration time larger than the value calculated by the iteration method and those of other hydrophobic compounds, such as phenanthrene, fluoranthrene, and Sudan III (Table 1). Methanol and ethanol were used as tracers for the aqueous phase. 

Preparation and Polymerization of (0/W) Cetyltrimethylammonium Bromide Microemulsion (CTAB-yE) (5-7). An oil in water pE composed of 1.0 g of CTAB, 0.5 g of hexanol, and 1.0 g of 50% styrene-divinylbenzene in 50 mL of water was carefully prepared by slowly adding the water to a stirred mixture of the other components to yield a slightly bluish clear solution. A 0.1% solution (w/w) of initiator AIBN (based on monomer) was then solubilized in the system followed by removal of 02 (by gentle N2 bubbling for 5 min), and finally the system was heated in an oil bath (50°C) until complete polymerization was achieved as determined spectrophotometrically. Proper dilution with water was then made to give a 0.01 M CTAB-P-pE solution P-pE indicates polymerized microemulsion. 

The properties of a microemulsion will to a great extent depend on the nature of the surfactant. Surfactants may be non-ionic, anionic or cationic. Previous studies have shown that a suitable system for the preparation of metallic nanoparticles consists of a non-ionic surfactant such as pentaethyleneglycol-dodecylether, hexane and water (Figure 2). A water-soluble precursor can be added to the system and thus, a reasonable amount of nanosized metal particles may be obtained. In some particular cases, systems based on ionic surfactants such as AOT or cationic surfactants such as cetyltriammonium bromide (CTAB) will give a lower solubility of the metal precursor. ... 

Based on the NMR and UV-Vis spectrophotometric results, the following models can be proposed for the four systems investigated (Fig. 8). Both Co(II) and Ni(II) atoms are retained at the interface in the different systems. More than one hexanol molecule enters the first coordination shell of Ni(II) ions. Co(II) interacts with one hexanol molecule in the CTAB-hexanol-water microemulsions, whereas both decanol and Triton X-100 molecules enter its first coordination shell. The Fe(III) ions are strongly hydrated in the inner water cores, and no hexanol molecules are able to replace the strongly held water molecules with this highly charged ion. Finally, the CTA ions interact indirectly only with the positively charged complexes. 

Only one study was found on microemulsion-based preparation of zinc oxide [103]. The two-microemulsion synthesis protocol was used by Hingorani et al. [103] to prepare zinc carbonate that was then calcined ( 220 C) to produce zinc oxide. Working with the CTAB/butanol/octane/water microemulsion system and the two-microemulsion protocol, one aqueous pseudophase contained zinc nitrate while the other contained ammonium carbonate. X-ray diffraction identified the resulting calcined particles as ZnO with an average particle size of 14 nm. 

The kinetic studies performed on the polymerization of MMA in the emulsion and microemulsion regions of ternary systems based on cetyltrimethylammonium bromide (CTAB) confirmed the difference in mechanisms between the two processes [84,91]. The following kinetic laws were obtained ... 

Antonietti and Nestl [88] reported a study using a new class of metallosurfactants that allowed them to reduce both particle size and surfactant concentration. Figure 11 shows the variation of the hydrodynamic radius of polystyrene particles as a function of the weight ratio of surfactant to monomer SIM) for microemulsions based on a classical surfactant, cetyltrimethylammonium chloride (CTAB) and the metallosurfactant tetradecyl-diethanolamine copper (TDEA-Cu). With this class of surfactants, the authors succeeded in getting a particle diameter as low as 14 nm (width of the distribution = 0.38), with an SIM value of 3. This results in a considerable surface area ( 500 m /g), which renders these systems of interest for subsequent functionalization. 

Using just one surfactant without cosurfactant, it has been possible to prepare latex particles as small as about 20 nm in diameter [116]. They have used CTAB, or a mixture of anionic and nonionic surfactants. These systems have been more thoroughly studied as shown in a recent review by Antonietti [117]. This author [118] and later Vu [119] were able to develop a predictive model for styrene microemulsion polymerization with crosslinker, initiated by AIBN. This model shows that the size of the droplets is dependent on the ratio between the weight fraction of monomer and the total amount of monomer plus surfactant. This model is based on simple geometrical considerations, the monomer mixture being the core of a particle surrounded by a shell of surfactant. 

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