66 research outputs found
NMR study of mixed micelles: zwitterionic – cationic surfactant systems
No full textSurfactant molecules in a solvent self-associate into various kinds of supramolecular assemblies such as micelles, vesicles, and liquid crystals and their mixture, especially those of nonionic and ionic surfactants are used in many practical applications, such as detergents, cosmetics, oil recovery, drug delivery systems, emulsified polymerization, coating technology, and mesostructured nanofilms (1). For these applications, the structural and solution properties of the mixed surfactant systems should be controlled effectively. Therefore, it is useful to understand how the molecular structures of surfactants in mixtures affect the solution properties, such as the size, shape, and surface charge density of the mixed micelles. For these reasons, structural properties of nonionic-ionic surfactant mixed micellar solutions have been investigated theoretically and experimentally (2). In the mixture of two or more different surfactants (nonionic and ionic), the complex aggregation behavior of the mixture of surfactants in solution is a result of a delicate balance of opposing forces, i.e., the steric hindrance among the polar head groups of the surfactant molecules and electrostatic repulsion energy between charges on the polar head of the ionic surfactant molecules (3). Therefore, the structural properties of the nonionic-ionic mixed micellar solutions should be studied as a function of the molar ratio to determine the effect of molecular interaction between the surfactants in a mixed micelle on its formation. Practically, this understanding can help in choosing relevant surfactant structures that will result in the desired properties. NMR spectroscopy is one of the most convenient methods for simultaneous monitoring of changes in aggregate morphologies of interaction between components.
In this study, we investigated the formation in water of mixed micelle using zwitterionic and anionic surfactants employing multinuclear NMR to study the influence of intramicellar interaction and surfactant molecular shape on the properties of mixed micelles.
In our experiments, we kept the surfactant concentration well above their cmc values, so the observed chemical shifts are those of aggregated assemblies formed upon mixing of the surfactants.
Interestingly enough, NMR experiments suggest that under the chosen experimental conditions upon mixing of pure surfactants two different families of mixed aggregates are formed both larger than the original single component micelles. The fact that the different mixed micelles coexist unchanged many days after solution preparation, suggest that the system is under thermodynamic control
Supramolecular Assemblies as Promoters of Iodohydrin Formation
No full textFinding alternative reaction media to replace polluting organic solvents is one aim of green chemistry. The ultimate green solvent, water, is the cheapest, non-toxic and most readily available reaction medium: three properties which make it an environmentally and economically attractive solvent. However, a fundamental problem in performing reactions in water is that many organic substrates are hydrophobic and not soluble in water. Several approaches are possible in solubilizing these compounds in aqueous media, one of which is carrying out reactions in aqueous solutions of surfactants at concentrations above their critical micellar concentration (cmc). Reactions of iodine with cyclohexene, 1-octene and styrene in water or in the presence of cationic surfactants do not give useful amounts of iodohydrins, but reactions in anionic surfactants give good yields. Iodohydrins are important functionalizable compounds and are readily prepared in the presence of sodium dodecyl sulfate (SDS) or sodium N-dodecanoyl sarcosinate (SANa). The critical conditions for these reactions were optimized with a rigorous statistical approach, the experimental design method. Use of these newly optimized reaction conditions gave high yields in short times for all of the alkenes examined. The use of anionic surfactants in water to form iodohydrins is a valid alternative to methods previously described
Reverse Micellar Aggregates: Effect on Ketone Reduction. 2. Surfactant Role
No full textKinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH4) were followed by UV−vis spectroscopy at 27.0 °C in different reverse micellar media, toluene/BHDC/water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH4 concentration and the amount of water dispersed in the reverse micelles, W0 = [H2O]/[surfactant]. In all cases, the first-order rate constant, kobs, increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH4- is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH4- being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, kobs increases with W0 in the whole range studied while in AOT the kinetic profile has a maximum at W0 5, probably reflecting the fact that BH4- is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W0 = 5 and, above that, BH4- is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W0 < 10, it was considered that NaBH4 is wholly at the interface and, at W0 ≥ 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH4 distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface
Surfactant control of the ortho/para ratio in the bromination of anilines. 4
No full textThe bromination of N,N-disubstituted anilines was performed in aqueous suspension of 1-hexadecylpyridinium tribromide (CPyBr3), using the
surfactant counterion as reagent. The experiments carried out at room temperature showed the same regioselectivity observed in homogeneous
solutions, with the prevalence of the para-bromo product. On the contrary, at 0 ◦C the [ortho-bromo]/[para-bromo] ratio increased. This result can
be ascribed to the location of the aniline at the solid–water interphase and to the tight packing at low temperature
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