Rheological properties of wormlike micelles formed in concentrated region of nonionic surfactant (C16E7)-water system

Rheological measurements have been made in concentrated micellar solutions (up to 40 wt%) of heptaethylene glycol n-hexadecyl ether (C16E7)/D2O system. The stress relaxation time (τ) and the plateau modulus (G∞) have been obtained from the frequency dependences of G′ and G″ (viscoelastic spectra), and the zero-shear viscosity (η0) has been obtained from the shear rate dependence of the steady shear viscosity. The slope of the double logarithmic plot of G∞ vs. concentration is almost independent of temperature whereas that of τ steeply decreases with increasing temperature and becomes negative at 40 °C. The slope for η0 also steeply decreases with increasing temperature and becomes almost constant above about 50 °C. These limiting values at higher temperatures are in good agreement with the theoretical values predicted by a simple model of Hoffmann (ACS Symp Ser 578:2–31 (1994)) where the ghost-like crossing and/or sliding connections are taken into account for the stress relaxation. Comparison with the recent theory of Tanaka (Langmuir 26:5374 (2010)) has also been made. The present results confirm the conclusion of the self-diffusion study of Kato et al. (Langmuir 11:4661 (1995)); the crosslinking between wormlike micelles begins to form above about 45 °C, and almost saturated networks are formed above about 55 °C.