Effect of ionic strength on rheological behavior of polymer-like cetyltrimethylammonium tosylate micellar solutions

Abstract

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, K₂SO₄ or K₃PO₄) 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 (σ/G₀, being σ and G₀ the shear stress and the plateau modulus, respectively) that increases with salt content and counterion valency. The zero-shear viscosity (η₀) and the main relaxation time (τ_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, G₀ 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.