Surfactants stabilizing

Some phosphorus-containing surfactants have bacteriostatic properties. In combination with their physiological acceptance they are used in cosmetics and pharmaceuticals. In mutanase oral anticarie compositions the combination of alkyl phosphates and nonionic surfactants stabilizes mutanase. A dentifrice useful for prevention of dental caries contains 0.5% sodium lauryl phosphate and 1.5% polyoxyethylene (20) sorbitan monooleate [222]. 

Solutions of surfactant-stabilized nanogels share both the advantage of gels (drastic reduction of molecular diffusion and of internal dynamics of solubilizates entrapped in the micellar aggregates) and of nonviscous liquids (nanogel-containing reversed micelles diffuse and are dispersed in a macroscopicaUy nonviscous medium). Effects on the lifetime of excited species and on the catalytic activity and stability of immobilized enzymes can be expected. 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

Scheme 5 Hydrogenation of anisole by surfactant-stabilized Rh(0) and lr(0) catalysts...

The selectivity in the hydrogenation of di-substituted benzenes such as xylene, methylanisole, cresol was also reported. In all cases, the czs-compound is largely the major compound > 80%. The ratio cis/trans decreases with the position of the substituents o > m>p but the identity of the metal does not seem important with this surfactant-stabilized system [45,47]. 

The catalytic applications of Moiseev s giant cationic palladium clusters have extensively been reviewed by Finke et al. [167], In a recent review chapter we have outlined the potential of surfactant-stabilized nanocolloids in the different fields of catalysis [53]. Our three-step precursor concept for the manufacture of heterogeneous egg-shell - nanocatalysts catalysts based on surfactant-stabilized organosols or hydrosols was developed in the 1990s [173-177] and has been fully elaborated in recent time as a standard procedure for the manufacture of egg-shell - nanometal catalysts, namely for the preparation of high-performance fuel cell catalysts. For details consult the following Refs. [53,181,387]. 

Figure 2. Schematic illustrations of (a) polymer-stabilized, (b) surfactant-stabilized, (c) ligand-stabilized metal nanoparticles.

W. A. Rendall, C. Ayasse, and J. Novosad. Surfactant-stabilized foams for enhanced oil recovery. Patent US 5074358, 1991. 

L. L. Schramm, C. Ayasse, K. Mannhardt, and J. Novosad. Method for improving enhanced recovery of oil using surfactant-stabilized foams. Patent CA 2006482,1991. 

A. Nandi, A. Mehra, D. V. Khakhar 1999, (Suppression of coalescence in surfactant stabilized emulsions by shear flow), Phys. Rev. Lett. 83, 2461. 

ThF Tadros. Polymeric surfactants , stabilization of emulsions and dispersions. In E Desmond Goddard, JV Gruber, eds. Principles of Polymer Science and Technology in Cosmetics and Personal Care. New York Marcel Dekker, 1999, pp 73-112. 

The organic phase transfer was observed for surfactant-stabilized Rh(0) NPs previously synthesized in an aqueous solution of N,N-dimethyl-N-dodecyl-N-(2-hydroxyethyl)ammonium chloride (HEA12.C1) [60]. The addition of LiNTf2 to the aqueous suspension of Rh-HEA12.C1 transferred the hydrosol NPs to the IL phase (HEA12.NTf2). 

Stone, M.T., Smith, P.G., da Rocha, S.R.P., Rossky, P.J. and Johnston, K.P. (2004) Lowinterfacial free volume of stubby surfactants stabilizes water-in-carbon... 

Mevellec, V., Roucoux, A., Ramirez, E., Philippot, K. and Chaudret, B. (2004) Surfactant-stabilized aqueous iridium(O) colloidal suspension an effident reusable catalyst for hydrogenation of arenes in biphasic media. Advanced Synthesis and Catalysis, 346 (1), 72-76. 

Brown D, Croudance CP, Williams NJ, Shearing JM, Johnson PA (1998) The effect of phthalate ester plasticisers tested as surfactant stabilized dispersions on the reproduction of the Daphnia Magna. Chemosphere 36 1367-1379... 

Potential applications of vitamin Bi2 in electrocatalytic degradation of dibromide and a-haloacetic acid pollutants has been demonstrated in aqueous buffers303,304 and in surfactant-stabilized emulsions.305 Electroreductive dehalogenations in water and microemulsions were also efficiently catalyzed by a vitamin Bi2 derivative grafted onto a polylysine-coated electrode.306... 

Typical approaches to this biphasic system have involved the immobilization of catalysts in the aqueous phase as colloids [53] or using water-soluble catalysts based on ligands such as the trisulfonated TPPTS [55, 56]. Particularly high reaction rates have been obtained with surfactant-stabilized microemulsions and emulsions that allow for intimate contact of all reagents with the catalyst during the reaction [57]. The emulsions separate readily into two phases by small pressure changes and the C02-phase is then vented to isolate the products. The catalyst RhCl(tppds)3 (tppds =... 

Toshima et al. obtained colloidal dispersions of platinum by hydrogen- and photo-reduction of chloroplatinic acid in an aqueous solution in the presence of various types of surfactants such as dodecyltrimethylammonium (DTAC) and sodium dodecylsulfate (SDS) [60]. The nanoparticles produced by hydrogen reduction are bigger and more widely distributed in size than those resulting from the photo-irradiation method. Hydrogenation of vinylacetate was chosen as a catalytic reaction to test the activity of these surfactant-stabilized colloids. The reaction was performed in water under atmospheric pressure of hydrogen at 30 °C. The photo-reduced colloidal platinum catalysts proved to be best in terms of activity, a fact explained by their higher surface area as a consequence of their smaller size. 

Similar surfactant-stabilized colloidal systems have been reported by Albach and Jautelat, who prepared aqueous suspensions of Ru, Rh, Pd, Ni nanoparticles and bimetallic mixtures stabilized by dodecyldimethylammonium propane-sulfonate [103]. Benzene, cumene and isopropylbenzene were reduced in biphasic conditions under various conditions at 100-150 °C and 60 bar H2, and TTO up to 250 were obtained. 

Scheme 9.10 Hydrogenation of anisole by reusable surfactant-stabilized lr(0) colloids in water. TEM micrograph and size distribution of Ir(0). (Adapted from [102].)...

Equation 3.1 and Equation 3.2) (including the mean particle sizes obtained) (Adapted from Bonnemann, H. and Brijoux, W., in Surfactant-Stabilized Nanosized Colloidal Metals and Alloys as Catalyst Precursors/Advanced Catalysts and Nanostructured Materials, Moser, W., Ed., Academic Press, San Diego, 1996, pp. 165-196, Chap. 7. With permission from Elsevier Science.)... 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

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