Scattering from Short Stiff Cylindrical Micelles Formed by Fully Ionized TDAO in NaCl/Water Solutions Vasil M. Garamus,* ,† Jan Skov Pedersen, ‡ Hiroshi Maeda, § and Peter Schurtenberger | GKSS Research Centre, Max-Planck Strasse, 21502 Geesthacht, Germany, Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark, Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 812-8581, Japan, and Department of Physics, University of Fribourg, Perolles, 1700 Fribourg, Switzerland Received December 2, 2002. In Final Form: February 18, 2003 Aqueous solutions of fully ionized tetradecyldimethylamine oxide (TDAO) with addition of 0.1 M NaCl were investigated by small-angle neutron scattering (SANS) and static light scattering (SLS). The analysis of the data showed that the surfactant under these conditions forms short stiff cylindrical micelles. The surfactant concentration ranges from 2 times the critical micelle concentration to well above the overlap concentration of the cylindrical micelles. A self-consistent approach was used for analyzing the SANS and SLS data simultaneously. The forward scattering was determined from the scattering at low scattering vectors, and from this, the apparent molar mass of the micelles was derived. It was fitted by a model with a power-law growth law combined with a term which describes intermicellar interaction effects. The length of the micelles was derived from the growth law using the mass per unit length as determined from the scattering data at high scattering vectors. Finally, the data in the full range of measured scattering vectors were fitted for all concentrations by a model based on the polymer reference interaction site model with a simple ansatz for the direct correlation function. The variations of the parameters of the micelles as obtained in the analysis were compared to the parameters for solutions of half-ionized TDAO for which semiflexible micelles are formed. 1. Introduction The formation of long micelles is observed in solutions of ionic surfactant under certain conditions. 1-3 These micelles are generally formed in the presence of a high salt concentration. The physical properties of such systems are of great interest, because a small change in the surfactant molecular structure, the type of counterion, or the amount of added salt may alter the size, flexibility, and interactions of the micelles. This, in turn, gives marked effects on the macroscopic rheological properties. The relationship between the flexibility of micelles, which can be characterized by the persistence l p or Kuhn length b, and the rheological properties is yet not known, and it is not possible to predict rheology a priori. There are currently a lot of efforts to study the above-mentioned relationship. 2-4 Some of the results of these studies are that the linear rheological properties (“Maxwell-like”) can be scaled with concentration 4-7 and the increase of fluidity is due to the transformation from linear micelles to branched systems. 4,8 There are still open questions such as, what is the origin of the maximum of the zero shear viscosity as a function of the surfactant concentration and salt concentration? There has been significant progress during the last years in the study of the dependence of the flexibility of micelles on the salinity of the solutions. 9-14 It has been clearly shown that flexibility increases with salt addition. The persistence length is conventionally described as being the sum of two contributions: an intrinsic contribution, as one has for neutral chains, and an electrostatic contribution. Even at a high concentration of salt (2 M), the electrostatic contribution to the persistence length is detected in the case of the anionic surfactant sodium dodecyl sulfate (SDS). 9 In the case of nonionic surfactants or ionic surfactants in high-salt solutions, the growth of elongated micelles versus surfactant concentration is predicted by mean- field theory 1 to have a simple power-law form, that is, 〈L〉 w ∼ c 0.5 , where 〈L〉 w is the weight-average length of the micelles in dilute solutions and 〈L〉 w ∼ c 0.6 in the semidilute regime. A lot of attempts to experimentally verify the mean-field theory predictions have been made by means of rheological techniques and scattering methods. The results are quite different and strongly influenced by the procedure used for analyzing the data. Rheological * Corresponding author. Phone: +49 4152 871290. Fax: +49 4152 871356. 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