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 cetyltrimethylam monium-based surfactants (CTAX): cetyltrimethylammonium n-hexane sulfo nate (CTAC(6)SO(3)), cetyltrimethylammonium n-heptane sulfonate(CTAC(7 )SO(3)), and mixtures of cetyltrimethylammonium 3-hydroxy-2-naphthalen ecarboxylate (CTAHNC) and cetyltlimethylammonium bromide (CTAB) in the molar ratio 1:1 and 2:1. Both linear and nonlinear viscoelasticity pr operties have been investigated as a function of surfactant volume fra ction and temperature. For the system CTAHNC + CTAB (1:1), the effect of added salt (NaBr) was also studied. In the concentration and temper ature ranges used in this study, all the systems exhibit a viscoelasti c behavior characteristic of wormlike micellar solutions. By changing the lipophilic character of the counterion, one varies the net electri cal charge carried by the micelle, which allows us to present a genera l description of the effect of the electrostatic interactions on the v iscoelasticity of micellar systems.