Formation viscoelastic micellar solutions

As already stated, several studies have been pubUshed using nonionic surfactant systems. The formation of viscoelastic micellar solutions in mixed nonionic systems is interesting in basic research-as the relation between packing constraints of hydrophobic chains and micellar growth would be clarified since the complicated interaction between the counterion and headgroup does not occur-as well as in applications such as cosmetics or pharmacy, where the avoidance of ionic additives is often desirable. 

Kunieda s group reported numerous viscoelastic worm-like micellar systems in the salt-free condition when a lipophilic nonionic surfactant such as short hydrophilic chain poly(oxyethylene) alkyl ether, C EOni, or N-hydroxyethyl-N-methylaUcanolamide, NMEA-n, was added to the dilute micellar solution of hydrophilic cationic (dodecyltrimethylammonium bromide, DTAB and hexade-cyltrimethylammonium bromide, CTAB) [12-14], anionic (sodium dodecyl sulfate, SDS [15, 16], sodium dodecyl trioxyethylene sulfate, SDES [17], and Gemini-type [18]) or nonionic (sucrose alkanoates, C SE [9, 19], polyoxyethylene cholesteryl ethers, ChEO [10, 20], polyoxyethylene phytosterol, PhyEO [11, 21] and polyoxyethylene sorbitan monooleate, Tween-80 [22]) surfactants. The mechanism of formation of these worm-Hke stmctures and the resulting rheological behavior of micellar solutions is discussed in this section based in some actual published and unpublished results, but conclusions can qualitatively be extended to aU the systems studied by Kunieda s group. 

Acharya, D.P., and Kunieda, H. (2003) Formation of viscoelastic wormlike micellar solutions in mixed nonionic surfactant systems. J. Phys. Chem. B, 107, 10168-10175. 

Effect of C12EOn mixed-surfactant systems on the formation of viscoelastic wormlike micellar solutions in sucrose alkanoate- and CTAB-water systems. Colloid Surf A, 279,113-120. 

A model consisting of the codeformational MaxweU constitutive equation coupled to a kinetic equation for breaking and re-formation of micelles is presented to reproduce most of the nonlinear viscoelastic properties of wormlike micelles. This simple model is also able to predict shear banding in steady shear and pipe flows as well as the long transients and oscillations that accompany this phenomenon. Even though the model requires six parameters, all of them can be evaluated from single and independent rheological experiments, and then they can be used to predict other flow situations. The predictions of our model are compared with experimental data for aqueous micellar solutions of cetyltrimethylammonium tosilate (CTAT). 

Another type of micelle formation has also attracted the attention of researchers, a brief mention of which will be made below. Gravsholt [93], in an early work, reported viscoelasticity in highly diluted aqueous solutions of some cationic surfactants, namely, certain derivatives of the base structure of cetyltrimethylammonium bromide (CTAB), CTA-X (where X=salicylate, m-chlorobenzoate, p-chlorobenzoate). He came to the conclusion that the viscoelastic behavior indicates a micellar shape other than spheres and rods. Later it has been shown [e.g. 94] by fluorescence anisotropy using fluorescent probe molecules that CTAB and sodium p-toluenesulfonate, NapTS (with suitably weak fluorescence) in aqueous solution could produce long, threadlike micelles with network structure in which the cross-points had finite lifetime. Sodium salicylate was another useful additive for synthesis, but the intensity of its fluorescence was found unsuitable for examining the behavior of the probes. 

Ubnius, J., Wennerstroem, H., Johansson, L.B.-A., Lindblom, G., Gravsholt, S. Viscoelasticity in surfactant solutions. Characteristics of the micellar aggregates and the formation of periodic colloidal structures. J. Phys. Chem. 1979, 53(17), 2232-2236. 

The results obtained in this study clearly show the complexity of the size and shape of CTAB micelles in KBr solution with the addition of benzyl alcohol. A small amount of alcohol solubilized in the interfacial region of the aggregates renders rodlike micelles larger and longer. As a result of this process, the viscosity of the dilute surfactant systems will rise and the viscoelasticity of concentrated solutions will increase due to the formation of a network structure of the micelles. When the alcohol content is higher, it will be solubilized in the palisades of the micelles and the rodlike micelles transform gradually into smaller oblate spheroid ones. Both the viscosity of dilute surfactant systems and the viscoelasticity of concentrated micellar systems will decrease. 

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