Contrasting the Effects of Hydrophobicity and Counterion Size on Anionic Wormlike Micelle Growth

Rheological measurements were performed to understand the effect of benzyltrimethyl ammonium bromide (BTAB) and NaBr on the growth of wormlike micelles formed by sodium oleate (NaOA). Both salts can make the micellar solution viscoelastic, and the rheological responses verify the formation of wormlike micelles. The viscoelastic solution follows the Maxwell model of a single stress relaxation mode in the low-frequency region. In comparison, the BTAB system exhibits stronger viscoelasticity than the NaBr system at constant salt concentration, but the critical overlapping concentration shows no significant difference; less BTAB can induce the solution to be more viscous than NaBr, but the viscosity maximum of the BTAB system is remarkably lower than that of NaBr at fixed NaOA content. The puzzling result is attributed to the effect of the composition on the packing parameter. In addition, it is shown that the zero-shear viscosity of the two salt systems decreases upon heating, following the Arrhenius mode well.