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The dehydrobromination reaction 2-(p-nitrophenyl)ethyl bromide + OH− was investigated in several alkanediyl-a-w-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br− (with s = 2, 3, 4, 5, 6, 8, 10, 12) micellar solutions, in the presence of NaOH 5 × 10−3 M. The kinetic data were quantitatively rationalized within the whole surfactant concentration range by using an equation based on the pseudophase ion-exchange model and taking the variations in the micellar ionization degree caused by the morphological transitions into account. The agreement between the theoretical and the experimental data was good in all the dimeric micellar media studied, except for the 12-2-12,2Br− micellar solutions. In this case, the strong tendency to micellar growth shown by the 12-2-12,2Br− micelles could be responsible for the lack of accordance. Results showed that the dimeric micelles accelerate the reaction more than two orders of magnitude as compared to water.

Effects of ethylene glycol (EG) addition on the micellization and on the micellar growth in two aqueous didodecyl dicationic dibromide surfactant, 12-s-12,2Br−(s = 2, 6) solutions, with the weight percentage of EG up to 50%, have been investigated. An increment in the amount of EG makes the aggregation process less spontaneous due to the water−EG mixtures being better solvents for the cationic dimeric surfactant molecules than pure water (solvophobic effect). Results show that C*, the surfactant concentration where the sphere-to-rod transition occurs, increases when EG content in the bulk phase increases. The amount of the organic solvent influences C* principally through the decrease in the hydrocarbon/bulk phase interfacial tension (air/bulk phase surface tension) caused by its presence. Changes in the aggregation number, in the micropolarity, in the microviscosity, and in the rheological behavior accompanying micellar growth were studied in the water−EG micellar solutions. Kinetic studies provide information about the characteristics of the dimeric micelles as microreactors. Kinetic data also show that an increase in the surfactant concentration leads to micellar growth. [ABSTRACT FROM AUTHOR]

Micellization of several surfactants in water−organic solvent mixtures has been investigated. Only solvents localized mainly in the bulk phase of the micellar solutions (they do not incorporate into the micelles) were studied, with either higher or lower permittivity than that of pure water. Results show that the influence of organic solvent addition on the aggregation process can be approximately accounted for by considering the changes in the bulk phase cohesive energy density, described by the Gordon parameter, G. To our knowledge, this is the first time that, for a given surfactant, the Gibbs energies of micellization, Δ GM°, obtained in several water−organic solvent mixtures have been fitted together. It is worth noting that data from different research groups have been considered. The Δ GM° versus Gcorrelation will permit the estimation of the variations in the Gibbs energy of micellization upon addition of known quantities of a given polar organic solvent. Speaking in a general way, organic solvent addition results in the bulk phase becoming a better solvent for the surfactant molecules. This would make the hydrophobic tail transfer from the bulk phase into the micelles less favorable and, as a consequence, Δ GM° increases (becomes less negative), making the aggregation process less spontaneous. [ABSTRACT FROM AUTHOR]

Critical micelle concentrations of the C mTAB+12- s-12 ( s= 3, 4, 5 and m= 10, 12, 14, 16) binary systems have been determined, through conductivity and fluorescence measurements, at 298 K. Application of different theoretical approaches to explain mixed micellization shows that non-ideality of the binary systems follows the trend C 16TAB+12-3-12 < C 14TAB+12-3-12 < C 12TAB+12-3-12 < C 10TAB+12-3-12, and C 12TAB+12-5-12 ≈ C 12TAB+12-4-12 < C 12TAB+12-3-12. Literature data corresponding to the C 12TAB+12-2-12, C 12TAB+ m- s- m( s= 2, 4, 6 and m= 12, 14, 16) and TritonX-100+12-s-12 (with s= 3, 6, 12) mixtures were considered in order to investigate the hydrophobic chain length and the spacer group length roles on the observed non ideal behavior. It was found that the capacity of the mixture components to form micelles of similar or different shapes plays a major role in non-ideality. [ABSTRACT FROM AUTHOR]

The micellization and micellar growth of cationic dimeric surfactant propanediyl---bis(dodecyldimethylammonium) bromide, 12-3-12,2Br-, have been studied in several water−organic solvent mixtures. The organic solvents were ethylene glycol, glycerol, 1,2-propylene glycol, 1,3-propylene glycol, acetonitrile, dioxane, formamide, and N,N-dimethylformamide. Results showed that the aggregation process was less favored in the binary mixtures than in pure water, which was explained by considering the influence of the solvophobic effect on micellization. The addition of organic solvents was accompanied by a diminution in the average aggregation number, Nagg, of the dimeric micelles. This diminution was due to the decrease in the interfacial Gibbs energy contribution, %@mt;sys@%%@ital@%G%@rsf@%%@sx@%interfacial%@be@%o%@sxx@%%@mx@%, to the Gibbs energy of micellization caused by the decrease in the hydrocarbon/bulk-phase interfacial tension. As a result of the micelle size diminution, the concentration at which the sphere-to-rod transition occurred, C, was higher in the mixtures than in pure water. Micelle size reduction is accompanied by a decrease in the ionic interactions and in the extra packing contribution to the deformation of the surfactants tails, making the formation of cylindrical micelles less favorable. [ABSTRACT FROM AUTHOR]