Micellization of dodecyltrimethylammonium bromide in water–dimethylsulfoxide mixtures: A multi-length scale approach in a model system

Abstract

The micellization in mixed solvent was studied using conductimetry, density measurements (molar volumes), and small angle neutron scattering (SANS) to explore dodecyltrimethylammonium bromide (DTABr) micelle formation throughout the entire composition range of water-dimethylsulfoxide (DMSO) mixtures. As the concentration of DMSO was increased in the mixture, the critical micelle concentration (CMC) increased, the aggregation number decreased and the ionization degree increased, until no aggregates could be detected any more for DMSO molar fraction higher than 0.51. The results were consistent with the presence of globular micelles interacting via a coulombic potential. The experimental CMC values and aggregation numbers were successfully reconciled with a molecular thermodynamic model describing the micellization process in solvent mixtures (R. Nagarajan and C.-C. Wang, Langmuir 16 (2000) 5242). The structural and thermodynamic characterization of the micelles agreed with the prediction of a dissymmetric solvation of the surfactant entity: the hydrocarbon chain was surrounded only by DMSO while the polar head was surrounded only by water. The decrease in the ionization degree was due to the condensation of the counterions and was definitely linked to the geometrical characteristics of the aggregates and by no means to the CMC or salinity. This multi-technique study provides new insight into the role of solvation in micellization and the reason for the decrease in ionization degree, emphasizing the dissymmetric solvation of the chain by DMSO and the head by water. This is the first time that, for a given surfactant in solvent mixtures, micellization is described using combined analysis from molecular to macroscopic scale.

Volumetric Properties of Water/AOT/Isooctane Microemulsions

“…The densities of the AOT/isooctane reverse micelles and the AOT/ water/isooctane microemulsions with R being 2, 8,10,12,16,18,20,25,30, and 40 were measured for various concentrations at 303.15 K and are listed in Tables S1 and S2 of the Supporting Information, respectively. Among these systems the density measurements of AOT/ isooctane reverse micelles and AOT/water/isooctane microemulsions with R being 20 were carried out in very low concentrations of the solute to explore the instability condition of the reverse micelles and the microemulsions.…”

“…In this paper, the micelle and microemulsions with AOT being the surfactant and isooctane being the oil and the molar ratios R being 2, 8,10,12,16,18,20,25,30, and 40 were investigated by density measurements at 303.15 K. We choose AOT as the surfactant in this study not only for its wide practical applications 20−23 but also for the large amount of information about it accumulated in the previous extensive studies. 24−27 The apparent specific volumes of AOT in the reverse micelle and the quasi-component water/AOT in the microemulsions with different R at various concentrations are deduced from the density data and used to examine the volumetric properties of the surfactant and water both in the droplets and in the continuous phase, to discuss the droplet interaction, and to determine the critical micelle or microemulsion concentrations.…”

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

“…E ij -van-der-Waals constant in kcal mol E el = Σ i Σ j q i q j /4πε r ij (9) q i -partial charge of the atom i, r ij -interatomic distance between atoms i and j, ε -dielectric constant…”

“…Therefore, an easy and reliable experimental determination of micellar aggregation number is of particular interest. A number of physical methods have been used for the determination of micelle aggregation number such as NMR spectroscopy [6,7], static light scattering (references in the text below), small-angle neutron scattering [8][9][10], small-angle X-ray scattering [11], fluorescence probing methods, and electron paramagnetic resonance [8,[12][13][14][15][16]. Some of them are limited to the aggregation number determination at the surfactant concentration equal to CMC which only provides aggregation number values for isolated non-interacting particles.…”

The Thermodynamic Properties of Dynol-604/AOT Mixed Reverse Micelles in Isooctane

“…properties of surfactants, both in the micellar and monomeric states [8][9][10][11][12][13]. However, only a few studies of the volumetric properties of reverse micelles [14], and no volumetric studies of mixed reverse micelles, have been reported.…”