Microemulsions preparation

The diastereoselection of the Diels Alder reaction of methyl acrylate with cyclopentadiene was investigated [74] in microemulsions prepared with isooctane oil, CTAB as surfactant and 1-butanol as cosurfactant, and the results were compared with those found in pure solvents and water (Table 6.12). In emulsions rich in 1-butanol and formamide (entries 1 and 4) the reaction was slow (72 h) and the diastereoselectivity was practically the same as that... 

Figure 1. Anionic surfactant aerosol OT for a microemulsion preparation.

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

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

The influence of surfactant structure on the nature of the microemulsion formed can also be predicted from the thermodynamic theory by Overbeek (17,18). According to this theory, the most stable microemulsion would be that in which the phase with the smaller volume fraction forms the droplets, since the osmotic term increases with increasing i. For w/o microemulsion prepared using an ionic surfactant, the hard sphere volume is only slightly larger than the water volume, since the hydrocarbon tails of the surfactant may interpenetrate to a certain extent, when two droplets come close together. For an oil in water microemulsion, on the other hand, the double layer may extend to a considerable extent, depending on the electrolyte concentration... 

Partitioning into oil droplets can be analyzed similarly as partitioning into micelles. Mrestani et al. (102) used microemulsions prepared from non-... 

Silica Gels. The acid-catalyzed alkoxide sol-gel process produces gels (17). Frib-erg and coworkers (40-50) pioneered the extension of this process to silica synthesis in microemulsions both aqueous and nonaqueous microemulsions were used. For aqueous microemulsions, experiments were conducted mostly with the SDS/ pentanol/water/acid system. A representative flow diagram is shown in Figure 2.2.9. The nonaqueous microemulsion systems utilized included CTAB/decanol/ decane/formamide and AOT/decane/glycerol (44-46,49,50). The experimental approach followed the sequence nonaqueous microemulsion preparation, water addition, and then TEOS addition. 

W/o microemulsions prepared from (i) pentaethyleneglycoldodecylether (PEDGE)-hexadecane or hexane-water, (ii) hexa-decyltrimethylammonium chloride (CTAC1)-octanol-water, and (iii) polyethylene sorbitanate tristearate (Tween)-hexane-decane-water... 

The solubilizing capacity of micelles and microemulsions prepared using the surfactant N,N-dimethyl-dodecylamine-N-oxide (C12AO), and containing either one of two semipolar ethyl ester oils (i.e., EH or EO) for the poorly water soluble, lipophilic steroidal drug, testosterone enanthate, has been determined recently (Barlow, D.J. Lawrence, M.J. Zuberi, T. Zuberi, S. Heenan, R.K. Personal communication, 2005). 

Human osteoblasts like cells osteosarcoma SaOS-2 cells were cultured for 24 hours, after which varying doses of a water dispersible microemulsion preparation of lycopene or vehicle of the same dilution were added. The cells were further cultured for 24 to 144 h and the cell numbers were counted. Lycopene at 10 and 10 M had significant stimulatory effects on cell numbers, compared with the corresponding vehicle treatment, at all time points from 24 h to 144 h. The effects of lycopene on activity of the differentiation marker alkaline phosphatase activity in the absence or presence of dexamethasone were shown to be dependent on osteoblasts of human origin [79]. 

Materials. Sodium dodecyl sulfate was supplied by Aldrich Chemical Company, Milwaukee, WI, and various alkyl alcohols were obtained from Supelco, Inc., Bellefonte, PA, with purity greater than 99%. For microemulsion preparation, sodium stearate (>> 99% pure) was supplied by Matheson, Coleman and Bell, Inc. and sodium myristate was supplied by K K Laboratories, Inc. All the oils (>> 99% pure) were obtained from Chemical Samples Company. Double-distilled water was used in all experiments. For monolayer studies, the 4.0 mM solutions of all pure alkyl alcohols were prepared in a mixture of methanol, chloroform and n-hexane in a volume ratio 1 1 3. 

Figure 1. Plots of the complex permittivity of the 0/W microemulsions prepared with Tween 60 on the complex dielectric plane ("Cole-Cole" plots), showing the depressed semicircles that indicate a distribution of relaxation times. Figure lb is an expanded portion of Figure la. A few frequencies are indicated for reference. Reproduced with permission from Reference 1. Copyright 1982 Academic Press.

Platinum catalysts were prepared by impregnation of formed supports with reverse microemulsions (water-in-oil) or water containing chloroplatinic acid. Microemulsion catalysts were more active in combustion of toluene in toluene-air mixtures than those prepared classically from water solutions. The size of platinum in classically impregnated catalysts was three times higher than that of the catalysts prepared from microemulsions. In the case of microemulsion preparation method, platinum is located near the pellet surface or its position in the pellet can be optimised. The viscosity of the microemulsion affects the concentration profile of platinum in the catalysts. 

Adding chloroplatinic acid to the w/o microemulsions prepared resulted in an impregnation solution useful for the preparation of catalysts with 0.3 wt. % Pt. 0-Alumina in the form of hollow pellets is a good example for studying the effect of viscosity on Pt concentration profiles (Fig. 2). 

Table 2 comprises important details concerning microemulsions preparation, characterisation of calcined catalysts and their activity in combustion of toluene expressed... 

With nonionic surfactants, both types of microemulsions can be formed, depending on the conditions. With such systems, temperature is the most cracial factor as the solubility of surfactant in water or oil is temperature-dependent. Microemulsions prepared using nonionic surfactants will have a limited temperature range. 

Sottmann, T., Lade, M., Stolz, M. and Schomacker, R. (2002) Phase behavior of nonionic microemulsions prepared from technical-grade surfactants. Tenside Surf. Det., 39, 20-... 

Grasset, F., Marchand, R., Marie, A.-M., Fauchadour, F. and Fajardie, F. (2006) Synthesis of Ce02-Si02 core-shell nanoparticles by water-in-oil microemulsion. Preparation of functional thin film. /. Colloid Interface Sci., 299, 726-Til. 

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. 

A study of the metal-support interaction effect has been carried out for a Pt-Ti02 catalyst prepared by microemulsion. Isomerization and cracking reactions of 2-methylpentane and hydrogenolysis of methylcyclopentane were chosen as model reactions for studying the influence of the catalyst nature on the strong metal-support interaction (SMSI) effect. A comparison between the cata-lyticbehaviour of similar catalysts prepared by microemulsion and incipient wetness method,was reported. The catalysts were reduced at 200 and 390 °C since it is well known that the reduction temperature plays a predominant role in the SMSI effect l The behaviour of thePt-TiO microemulsion-prepared catalyst was found to be similar to that of a Pt/AbOs catalyst. Also, it was found that the microemulsion-prepared catalysts require a higher temperature of reduction to induce metal-support interactions. 

CO hydrogenation was used as test reaction for the prepared catalysts. Pt, Pd and Rh microemulsion-based catalysts displayed higher activity that their impregnation counterparts. The activity for the microemulsion-prepared catalysts seems to increase with the particle size this was explained by taking into account that the smaller particles, obtained at low (o ratios, were buried inside the carrier, thus lowering the accessible rhodium sites. 

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