Shear-induced microstructure and rheology of cetylpyridinium chloride/sodium salicylate micellar solutions

Abstract

In this study, flow induced behavior of a micellar solution of cetylpyridinium chloride(CPC) and sodium salicylate was studied by employing a phase modulated flow birefringence and rheometry. We examined the shear viscosity, shear modulus and flow birefringence as a function of the surfactant and additive concentrations. In the presence of NaSal, the micellar solution exhibited the non linear viscoelastic behavior due to the formation of worm-like micelles when the molar ratio of NaSal was above 0.5. Also shown was that the shear viscosity increased until the molar ratio reached around unity. Meanwhile, the shear viscosity decreased as the molar ratio exceeded the optimum value. This implies that the scission of the micelle network junctions occurs at higher concentration of NaSal. In the optical experiment, flow birefringence and extinction angle were measured to investigate the change in the micelle orientation during flow. Intensity fluctuation appeared at the shear rate near the onset of shear thickening, which caused by shear-induced coagulation or aggregation. Near the optimum molar ratio, signals captured in on oscilloscope showed a slow response after the flow applied. The difference of I1w and I2w signals produced by changing the molar ratio can be explained by the relaxation time of each solution. At low shear rates, the flow birefringence increased as the shear rate increased. On the other hand, at higher shear rates, the flow birefringence decreased or was not reproducible due to the intensity fluctuation.