Optical and Rheological Properties of a Semi-Diluted Equimolar Solution of Cetyltrimethylammonium Bromide and Potassium Bromide

We report in this paper the results of rheological and optical experiments performed on a semi-diluted viscoelastic solution of cetyltrimethylammonium bromide (CTAB) in water with sodium bromide at a concentration of 0.3 M l-1 in surfactant and salt. This concentration in surfactant is much smaller than the concentration which gives a nematic phase at rest. The solution behaves like a Maxwell fluid with a single relaxation time. When submitted to a shear flow, this solution shows a behaviour typical of a system undergoing a phase transition. When observed between crossed polarizer and analyzer, the gap of the Couette cell appears divided in two bands the optical properties of which are different; in rheology, the curves σ(γ̇) are also characteristic of these systems where the shear stress presents a plateau extending between two critical values, γ̇_1 c, and γ̇_2 c of the shear rate. We finally compare the results with previous experiments performed on a concentrated solution of CTAB containing no salt.