Effect of ionic strength on rheological behavior of polymer-like cetyltrimethylammonium tosylate micellar solutions† E. R. Mac  ıas, a F. Bautista, b J. H. P erez-L opez, a P. C. Schulz, c M. Gradzielski, d O. Manero, e J. E. Puig a and J. I. Escalante * f Received 28th July 2010, Accepted 19th November 2010 DOI: 10.1039/c0sm00739k The influence of ionic strength on the rheological properties of polymer-like aqueous micellar solutions of cetyltrimethylammonium tosylate (CTAT) containing different salts (KCl, KBr, (COONa) 2 ,K 2 SO 4 or K 3 PO 4 ) is investigated. The rheological behavior of the solutions is analyzed above the concentration where a micellar entanglement network is formed, varying surfactant and salt concentration, salt counterion valency and temperature. A master curve of the linear viscoelastic properties is obtained by multiple superposition of time, temperature, salt type, and surfactant and salt concentration. Application of the existent kinetic theory provides information suggesting that the micellar solutions are in the fast breaking regime (i.e., the relaxation is kinetically controlled) regardless of salt type and concentration. Moreover, these solutions exhibit shear-banding flow with a reduced stress plateau (s/G 0 , being s and G 0 the shear stress and the plateau modulus, respectively) that increases with salt content and counterion valency. The zero-shear viscosity (h 0 ) and the main relaxation time (s C ) diminish with increasing salt content according to a step-like function, in which the number of steps increases with the salt counterion valence. In contrast, G 0 only increases slightly with increasing salt content for the five salts employed. These results are discussed in terms of ionic strength and screening of the electrostatic-interactions caused by the addition of salt. In addition, it was found that the influence of anions on the viscoelastic properties of the polymer-like micelles follows the Hofmeister series commonly encountered in macromolecular and biological systems. This finding opens a challenge for scientists in the experimental and theoretical fields. Introduction At concentrations larger than the critical micelle concentration (cmc), most surfactants spontaneously form spherical micelles in aqueous solutions; at even higher concentrations, they grow to yield rod-like micelles at the cmc II. 1 The addition of inorganic salts to ionic surfactant solutions screens the repulsions between the charged heads and decreases the concentration at which the transition from spherical to rod-like micelles occurs. 2–7 This transition takes place at even lower concentrations in the pres- ence of organic (hydrotropic) salts or in surfactants with coun- terions such as salicylate, 3,8–10 tosylate, 11 chlorobenzoate, 12 hydroxyl naphthalene carboxylate, 5,13 and alkyl sulfates 14 without the addition of electrolytes. The intensity of binding to the micelles is a function of the hydrophobicity of the coun- terion. 13 Most of the rheological studies of wormlike micellar solutions have been performed with cationic surfactants, such as alkyl- trimethylammonium halides and alkylpyridium halides, or anionic surfactants (sodium dodecyl sulfate), in the presence of inorganic or organic salts, i.e., in highly screened systems. 8,15–19 Some authors have added another inorganic salt, such as NaCl or KBr, to further screen the electrostatic interactions in these surfactant/salt (inorganic or organic)/water micellar systems. 16,20–22 By contrast, the structural and rheological reports on salt-free viscoelastic micellar solutions are scarce. Kern et al. 22 investi- gated the linear viscoelastic behavior of salt-free polymer-like a Departamento de Ingenier  ıa Qu ımica, Universidad de Guadalajara, Boul. M. Garc  ıa Barrag an # 1451, Guadalajara, Jal., 44430, M exico. E-mail: emmarebecamacias@hotmail.com; jperez50@hotmail.com; puigje@ hotmail.com b Departamento de F ısica, Universidad de Guadalajara, Boul. M. Garc ıa Barrag an # 1451, Guadalajara, Jal., 44430, M exico. E-mail: ferbautistay@yahoo.com c Departamento de Qu ımica, Universidad Nacional del Sur, Bah ıa Blanca, 8000, Argentina. E-mail: pschulz@criba.edu.ar d Stranski-Laboratorium f € ur Physikalische und Theoretische Chemie, Institut f € ur Chemie, Technische Universit € at Berlin, Straße des 17. Juni 124, Sekr. TC7, D-10623 Berlin, Germany. E-mail: michael.gradzielski@ tu-berlin.de e Instituto de Investigaciones en Materiales, UNAM, Apdo. Postal 70-360, M exico City, DF, 04510, Mexico. E-mail: manero@unam.mx f Departamento de Qu ımica, Universidad de Guadalajara, Boul. M. Garc  ıa Barrag an # 1451, Guadalajara, Jal., 44430, M exico. E-mail: escalant@ hotmail.com; Tel: +52-33-13785900 ext 7536 † Electronic supplementary information (ESI) available: Figs. S1 and S2. See DOI: 10.1039/c0sm00739k 2094 | Soft Matter , 2011, 7, 2094–2102 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2011, 7, 2094 www.rsc.org/softmatter PAPER Published on 21 December 2010. Downloaded by TU Berlin - Universitaetsbibl on 31/03/2016 14:37:47. View Article Online / Journal Homepage / Table of Contents for this issue