Origins of the Viscosity Peak in Wormlike Micellar Solutions. 1. Mixed Catanionic Surfactants. A Cryo-Transmission Electron Microscopy Study

Abstract

The rheology of wormlike micelles ("worms") formed by surfactants in water often follows nonmonotonic trends as functions of composition. For example, a study by Raghavan et al. (Langmuir 2002, 18, 3797) on mixtures of the anionic surfactant sodium oleate (NaOA) and the cationic surfactant octyl trimethylammonium bromide (OTAB) reported a pronounced peak in the zero-shear viscosity eta0 as a function of NaOA/OTAB ratio at a constant surfactant concentration (3 wt %). In this work, we study the origins of rheological changes in the NaOA/OTAB system and the relations between the composition and structural characteristics using cryo-transmission electron microscopy (cryo-TEM). When either surfactant is in large excess, the dominating morphology is that of spherical micelles. As oppositely charged surfactant is added to the mixture, the spheres grow into linear worms and these continue to elongate as the viscosity peak (which occurs at a 70/30 NaOA/OTAB ratio) is approached from either end. At the viscosity peak, the sample shows numerous long worms as well as a small number of branched worms. Taken together, NaOA/OTAB rheology can be primarily understood on the basis of micellar growth, which is explained primarily by packing arguments. While the size of the hydrophobic micellar core continuously decreases as the short amphiphile OTAB is added at the expense of NaOA, screening of charges goes through a maximum, which contributes to the asymmetry of the viscosity curve. With regard to micellar branching, there is no significant difference in the density of branched worms on either side of the viscosity peak. Therefore, it appears that in contrast to the behavior of some surfactant/salt systems, branching does not have a significant influence on the rheology of this mixed catanionic surfactant system. Instead, our data clearly indicate that the origin of the viscosity peak is linked with micellar growth and micellar shortening.

Effects of small ionic amphiphilic additives on reverse microemulsion morphology

“…An example hydrotrope is sodium benzoate, which may be considered as a "primitive" short chain surfactant. The presence of a bulky hydrotrope is recognised as being critical for micellar elongation, with these systems shown to produce substantial increases in solution viscosities [10]. Close investigation by NMR and computer simulations revealed that the hydrotrope counterions adsorb at the micellar interfacial region [11].…”

Viscoelastic properties of new mixed wormlike micelles formed by a fatty acid salt and alkylpyridinium surfactant

“…However, in mixed anionic / cationic surfactant systems it may also be due to the shortening of micellar chains as was demonstrated for sodium oleate / C 8 TAB micelles by cryo-transmission electron microscopy [22]. But one should note that the experiments in the last paper [22] were performed at constant total concentration of cationic and anionic surfactants and therefore the amount of long-chain anionic surfactant was decreased as the cationic short-chain surfactant was added, which may favor a decrease of micellar length. In contrast, in the present paper, we keep the anionic surfactant concentration constant.…”

1-Hexanol triggered structural characterization of the worm-like micelle to vesicle transitions in cetyltrimethylammonium tosylate solutions

“…NOESY experiments disclose that 1-hexanol resides close to palisade layer and below the head group inside CTAT micelles. Doping with 1-hexanol offers a simple and easy method of controlling the size and shape of CTAT micelles from worm-like to vesicles that nd many industrial applications as well as applications in personal and home care products and in templated synthesis of mesoporous materials 42,[79][80][81] and can also be utilized for micellar-enhanced ultraltration process.…”