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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]

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]