MICROSCOPIC ORIGINS OF PHOTORHEOLOGICAL EFFECTS INDUCED BY THE PHOTOISOMERIZATION OF STILBENE CARBOXYLIC-ACIDS IN AQUEOUS CETYLTRIMETHYLAMMONIUM BROMIDE SOLUTIONS

Aqueous micellar solutions containing cetyltrimethylammonium bromide (CTAB) and solubilized 3-stilbene carboxylic acid (3-SC) or 4-stilbene carboxylic acid (4-SC) known to exhibit photorheological effects were investigated via (i) oscillatory shear viscometry, (ii) static and dynamic light scattering and (iii) electrically induced birefringence. The presence of 3-SC and 4-SC induces the formation of swollen micelles as revealed from light scattering data. The viscosity increase observed upon in situ photoisomerization of 3-SC and 4-SC is accompanied by a further increase in micelle size as shown by light scattering data. Only solutions containing 3-SC at concentration ratios c(CTAB)/c3-SC less-than-or-equal-to 6.5 show non-Newtonian flow such as shear thinning, thixotropy and viscoelasticity, which are indicative of the presence of anisometric micelles. Photoisomerization induces non-Newtonian features in systems containing 4-SC and increases these features in 3-SC solutions. Two shear stress relaxation mechanisms are resolved from relaxation spectra.