Effect of the lipophilicity of the counterion on the viscoelasticity of micellar solutions of cationic surfactants

We have studied the effect of the lipophilicity of the counterions X- on the rheological properties-of aqueous solutions of cetyltrimethylammonium-based surfactants (CTAX): cetyltrimethylammonium n-hexane sulfonate (CTAC(6)SO(3)), cetyltrimethylammonium n-heptane sulfonate(CTAC(7)SO(3)), and mixtures of cetyltrimethylammonium 3-hydroxy-2-naphthalenecarboxylate (CTAHNC) and cetyltlimethylammonium bromide (CTAB) in the molar ratio 1:1 and 2:1. Both linear and nonlinear viscoelasticity properties have been investigated as a function of surfactant volume fraction and temperature. For the system CTAHNC + CTAB (1:1), the effect of added salt (NaBr) was also studied. In the concentration and temperature ranges used in this study, all the systems exhibit a viscoelastic behavior characteristic of wormlike micellar solutions. By changing the lipophilic character of the counterion, one varies the net electrical charge carried by the micelle, which allows us to present a general description of the effect of the electrostatic interactions on the viscoelasticity of micellar systems.