Microstructures and rheological responses of aqueous CTAB solutions in the presence of benzyl additives

Abstract

In this paper, we investigated the effects of various benzyl additives on the microstructure of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), in aqueous medium. Benzyl derivatives considered here were sodium salicylate (NaSal), sodium benzoate (NaBen), and p-methylsalicylic acid (PMSA). The CTAB concentration ranged from 1 to 20 mM, and the molar ratio of an additive to CTAB was varied from 0.1 to 10.0. The experimental results showed that the CTAB solution exhibited a variety of rheological responses, such as shear thinning, shear thickening, and long time relaxation, under an applied shear field. In particular, the shear viscosity of a dilute CTAB solution, which showed pronounced shear thinning at low shear rates, exhibited shear thickening at high sheer rates. As the shear rate was increased even more, the viscosity shear thinned again. The rheological response associated with microstructural transition was closely dependent on the concentrations and molecular structures of constituent substances as well as on temperature. The microstructural evolution was probed successfully by theological and rheo-optical measurements. Specifically, shear thickening of the aqueous CTAB solution and its microstructural transition were successfully monitored by the flow birefringence and the scanning electron microscope images under an applied shear flow. Finally, our results showed that hydrogen bonding between hydroxyl and carboxyl groups was responsible for long-chain structure formation of wormlike micelles. Consequently, the influence of benzyl derivatives such as NaSal, NaBen, and PMSA on CTAB solutions was characterized in terms of the position of the,hydroxyl group relative to that of the carboxyl group.