Effects of Protonation on the Viscoelastic Properties of Tetradecyldimethylamine Oxide Micelles

Marked effects of protonation (ionization) of tetradecyldimethylamine oxide on the viscoelastic properties of the micelle solutions were found. The effect strongly suggests the short-range attractive interaction between the headgroups of the nonionic (deprotonated) and the cationic (protonated) species. The zero shear viscosity reached a maximum at the half-ionized state (the degree of ionization α = 0.5) and the value was larger than that of the nonionic species (α = 0) or the cationic species (α = 1) by more than 2 orders of magnitude. At a surfactant concentration C of 0.1 mol/kg, approximately single Maxwell behavior was observed as α approached 0.5 from either side. For the half-ionized micelles (α = 0.5) in 0.1 mol/kg NaCl solutions, the steady-state compliance J e 0 decreased with C with an exponent of 2.1 ± 0.2, suggesting the presence of an entangled network of flexible threadlike micelles. The relaxation time, on the other hand, exhibited a nonlinear dependence on C. It was about 0.1 s and remained nearly constant in the range C > 0.1 mol/kg (regime I), whereas it increased with C in the range of C < 0.09 mol/kg (regime II) with an exponent slightly larger than 1. The single Maxwell behavior was observed in regime I. The regime shift was not controlled by the ratio C/m s, m s representing the NaCl concentration. Effects of NaCl concentration and the temperature on the viscoelastic properties were also examined at α = 0.5. Cryo-transmission electron micrographs clearly showed a highly entangled network in the solution for α = 0.5, while much smaller micelles for α = 0. Contrary to the expectation from the rheological results, a highly entangled network was also observed in the solution for α = 1.