ANOMALOUS BEHAVIOR OF MICELLAR SOLUTIONS IN ELECTRIC BIREFRINGENCE MEASUREMENTS

Aqueous solutions of surfactant systems containing rodlike micelles were examined by the transient and dynamic electric birefringence method. It is demonstrated that all surfactant solutions with charged anisometric micelles show an anomalous behavior in the electric field. The results were obtained on solutions of hexadecyloctyldimethylammonium bromide and on mixtures of tetradecyldimethylamine oxide and sodium dodecyl sulfate. In the dilute concentration range of both systems a normal birefringence is observed with a single relaxation time τ1, which is due to a rotational motion of the micelles. Beginning at the overlap concentration of the rods, a second effect with an opposite sign of the birefringence becomes apparent. The relaxation time τ2 of this effect is a factor 10-30 longer than τ1. At even higher concentrations a third effect τ3 with the same sign as the first one begins to appear. The amplitude of this effect increases rapidly with concentration and finally dominates the birefringence signal. The second effect is explained as a coupled rotational translational motion that is caused by an additional electric field of the electrically polarized surrounding micelles. During this process the rod axes are aligned perpendicular to the electric field. The third effect is due to a cooperative process of correlated micelles. In some cases a forth relaxation time τ4 can be observed, which seems to be due to a deformation and relaxation of a temporary network, which is built from the rods. The anomalous behavior is explained on the basis of intermicellar electrostatic interactions. The anomaly can be suppressed by addition of salt. Otherwise it is possible to charge a zwitterionic system like tetradecyldimethylamineoxide with sodium dodecyl sulfate molecules, and as a result the anomaly appears. © 1990 American Chemical Society.