STRESS OPTICAL-COEFFICIENT OF VISCOELASTIC SOLUTIONS OF CETYLTRIMETHYLAMMONIUM BROMIDE AND POTASSIUM-BROMIDE

The rheo-optical properties of different viscoelastic solutions of sur factant are investigated in order to gather experimental data which al low for the computation of the stress optical coefficient C. The surfa ctant which is the widely used cetyltrimethylammonium bromide (CTAB) i s mixed with potassium bromide, in various amounts in order to vary th e salinity of the solvent. Flow birefringence experiments and rheologi cal measurements are performed on these solutions in order to study th e dependence of the angle of extinction chi, of the birefringence inte nsity Delta n and of the shear stress sigma(yx) with the shear rate (g amma) over dot. These data are used to check the stress optical law wh ich turns out to be valid in a wide range of shear rates. The stress o ptical coefficient C is then computed: it is found to vary with the co ncentration of surfactant and with the salinity of the solvent.