  We report our studies of the linear and nonlinear rheology of aqueous
solutions of the surfactant cetyl trimethylammonium tosylate (CTAT) with
varying amounts of sodium chloride (NaCl). The CTAT concentration is fixed at
42mM and the salt concentration is varied between 0mM to 120mM. On increasing
the salt (NaCl) concentration, we see three distinct regimes in the zero-shear
viscosity and the high frequency plateau modulus data. In regime I, the
zero-shear viscosity shows a weak increase with salt concentration due to
enhanced micellar growth. The decrease in the zero-shear viscosities with salt
concentration in regimes I I and III can be explained in terms of
inter-micellar branching. The most intriguing feature of our data however, is
the anomalous behavior of the high frequency plateau modulus in regime II (0.12
$\le \frac{[NaCl]}{[CTAT]} \le$ 1. 42). In this regime, the plateau modulus
{\it increases} with an increase in NaCl concentration. This is highly
counter-intuitive, since the correlation length of concentration fluctuations
and hence the plateau modulus $G_{\circ}$ are not expected to change
appreciably in the semi-dilute regime. We propose to explain the changes in
regime II in terms of the unbinding of the organic counterions (tosylate) from
the CTA$^{+}$ surfaces on the addition of NaCl. In the nonlinear flow curves of
the samples with high salt content, significant deviations from the predictions
of the Giesekus model for entangled micelles are observed.
