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 ef fect strongly suggests the short-range attractive interaction between the h eadgroups of the nonionic (deprotonated) and the cationic (protonated) spec ies. The zero shear viscosity reached a maximum at the half-ionized state ( the degree of ionization alpha = 0.5) and the value was larger than that of the nonionic species (alpha = 0) or the cationic species (alpha = 1) by mo re than 2 orders of magnitude. At a surfactant concentration C of 0.1 mol/k g, approximately single Maxwell behavior was observed as a approached 0.5 f rom either side. For the half-ionized micelles (alpha = 0.5) in 0.1 mol/kg NaCl solutions, the steady-state compliance J(e)(0) decreased with C with a n exponent of 2.1 +/- 0.2, suggesting the presence of an entangled network of flexible threadlike micelles. The relaxation time, on the other hand, ex hibited 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 la rger than 1. The single Maxwell behavior was observed in regime I. The regi me 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 vi scoelastic properties were also examined at alpha = 0.5. Cryo-transmission electron micrographs clearly showed a highly entangled network in the solut ion for alpha = 0.5, while much smaller micelles for alpha = 0. Contrary to the expectation from the theological results, a highly entangled network w as also observed in the solution for alpha = 1.