Stress optical coefficient of viscoelastic solutions of cetyltrimethylammonium bromide and potassium bromide

The rheo-optical properties of different viscoelastic solutions of surfactant are investigated in order to gather experimental data which allow for the computation of the stress optical coefficient C. The surfactant which is the widely used cetyltrimethylammonium bromide (CTAB) is mixed with potassium bromide, in various amounts in order to vary the salinity of the solvent. Flow birefringence experiments and rheological measurements are performed on these solutions in order to study the dependence of the angle of extinction chi, of the birefringence intensity Delta n and of the shear stress sigma(yx) with the shear rate (gamma) over dot. These data are used to check the stress optical law which turns out to be valid in a wide range of shear rates. The stress optical coefficient C is then computed: it is found to vary with the concentration of surfactant and with the salinity of the solvent.