| Abstract
| - In this paper, we investigated the effects of various benzyl additives on the microstructure of a cationicsurfactant, cetyltrimethylammonium bromide (CTAB), in aqueous medium. Benzyl derivatives consideredhere were sodium salicylate (NaSal), sodium benzoate (NaBen), and p-methylsalicylic acid (PMSA). TheCTAB concentration ranged from 1 to 20 mM, and the molar ratio of an additive to CTAB was varied from0.1 to 10.0. The experimental results showed that the CTAB solution exhibited a variety of rheologicalresponses, such as shear thinning, shear thickening, and long time relaxation, under an applied shearfield. In particular, the shear viscosity of a dilute CTAB solution, which showed pronounced shear thinningat low shear rates, exhibited shear thickening at high shear rates. As the shear rate was increased evenmore, the viscosity shear thinned again. The rheological response associated with microstructural transitionwas closely dependent on the concentrations and molecular structures of constituent substances as wellas on temperature. The microstructural evolution was probed successfully by rheological and rheo-opticalmeasurements. Specifically, shear thickening of the aqueous CTAB solution and its microstructural transitionwere successfully monitored by the flow birefringence and the scanning electron microscope images underan applied shear flow. Finally, our results showed that hydrogen bonding between hydroxyl and carboxylgroups was responsible for long-chain structure formation of wormlike micelles. Consequently, the influenceof benzyl derivatives such as NaSal, NaBen, and PMSA on CTAB solutions was characterized in termsof the position of the hydroxyl group relative to that of the carboxyl group.
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