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The micellization and micellar growth in pure aqueous alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br−(with s= 2,5,6,8,10,12), and N-decanoyl-N-methylglucamide MEGA10 solutions and their mixtures are investigated at 303 K. Application of different theoretical approaches to the binary mixtures shows a nonideal behavior. It also shows that the spacer length does not play an important role in the attractive interactions shown by the mixed systems. The sphere-to-rod morphological transition in the pure dimeric micellar solutions is studied at 303 K. From comparison of these results with those at 298 K the key role played by the spacer in the micellar growth is shown. The spacer length controls not only the surfactant concentration at which the morphological transition happens but also the sign of the enthalpy change accompanying the sphere-to-rod equilibrium. Spacers with an even number of methylenes show smaller C* values than those with an odd number of −CH2− units. An endothermic enthalpy change is found for evenspacers whereas an exothermic enthalpy change is found for oddspacers. To the authors knowledge, this is the first time this experimental trend has been shown. Addition of MEGA10 diminishes the tendency of the aggregates to grow. An increment in the solution mole fraction of MEGA10 makes the formation of elongated micelles difficult. Microviscosity measurements provide additional information about the influence of the MEGA10 content on the sphere-to-rod transition. [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]