The sphere-rod transition of micelles of dodecyldimethylammonium bromide in aqueous NaBr solutions, and the effects of counterion binding on the micelle size, shape and structure

Abstract

Micelle size and shape of dodecyldimethylammonium bromide have been determined by measurement of light scattering from its aqueous NaBr solutions. In water and in the presence of NaBr up to 0.07 M, the Debye plots give straight lines with positive slopes, and spherical micelles having molecular weight less than 30 000 are formed. At higher NaBr concentrations, the Debye plots decrease with increasing micelle concentration, indicating the aggregation of the primary spherical micelles into larger secondary micelles. The molecular weight and the radius of gyration of the secondary miceltes increase with increasing NaBr concentration, and the relation between molecular weight and radius of gyration suggests that they are rodlike and flexible. Linear logarithmic relations between micelle molecular weight and ionic strength hold for spherical and rodlike micelles, respectively, and the threshold concentration of NaBr for the sphere-rod transition is located at 0.07 M. The spherical micelle of dodecyldimethylammonium ions has a size more than 20 surfactant ions larger in NaBr solutions than in NaCI solutions, and their rodlike micelle has a shorter length in NaBr solutions than in NaCI solutions, when compared at an identical aggregation number, indicating 2 more surfactant ions in its cross-section.

Effect of Temperature on Micelle Formation in Aqueous NaBr Solutions of Octyltrimethylammonium Bromide

“…The other one seems to be related to the change in the degree of counterion binding to the micelles caused by an increase in NaBr concentration. The decrease in the value of the dissociation degree, α m , of the micelles is more pronounced at lower salt concentrations (53,72,73). The increase in the average number of counterions bound to the surface of the micelle due to the addition of small quantities of NaBr to the aqueous surfactant solution should be accompanied by a large exothermic effect.…”

Determination of micelle aggregation numbers of alkyltrimethylammonium bromide and sodium dodecyl sulfate surfactants using time-resolved fluorescence quenching

“…The utilisation of this method for the aggregation number determination is rather complicated as it requires the independent determination of refractive index increment of the measured surfactant solution and the extrapolation of the data to the CMC which does not allow measurement of the concentration dependence of the aggregation number. Using this method, micelle aggregation numbers of some conventional singlechain ionic surfactants were determined at different solution ionic strength as published in the papers of Imae and Ikeda -dodecyltrimethylammonium bromide (DTAB) [17], dodecyldimethylammonium bromide [18] and chloride [19], dodecylpyridinium bromide [20], tetradecyltrimethylammonium halides [21], sodium dodecyl sulfate (SDS) [22,23]. Small angle neutron scattering data permit the determination of the average charge of micelles, the average micelle aggregation number [24] as well as providing information on the micelle shape.…”

Formation of rodlike micelles of dimethyloleylamine oxide in aqueous solutions: effects of addition of HCl and NaCl on the micelle size and the intermicellar interaction

“…Spherical micelles of ionic suffactants give the second virial coefficients of the order of 10 -3 cm 3 g-~ in 0.01 and 0.05 M NaC1 [24][25][26], and these values are reduced by further addition of NaC1. The second virial coefficient for a dimethyloleylamine oxide solution in pure water has a value of the same order as those for polyoxyethylene series [23].…”