Rheological properties of aqueous solutions of cetylpyridinium chloride in the presence of sodium chlorate

We have studied the effect of salt content on the linear viscoelastic properties of wormlike micelles formed from cetylpyridinium chloride in the presence of the nonpenetrating inorganic salt, sodium chlorate. Rotational rheometry and oscillatory squeeze flow were used to determine the shear moduli G′ and G″ in the angular frequency range from 0.1 rad s − 1 up to 104 rad s − 1. From G′ and G″ data, we deduce the zero-shear viscosity η0, relaxation time TR and plateau modulus G0. With regard to increasing salt concentration, at fixed surfactant concentrations of 0.3 and 0.6 M, we observe a maximum of η0 as well as of TR, whereas G0 increases continuously with ionic strength. This result is different from that obtained by Cappelaere and Cressely (Rheol Acta 39:346–353, 2000), who studied the same salt/surfactant system. They report a strong decrease of G0 with ionic strength at salt concentrations higher than that corresponding to the viscosity maximum and explained this decrease by a progressive diminution of the micellar length. We expect an inaccuracy of their oscillatory shear measurements in the high-frequency regime (100 < ω < 250 rad s − 1) to be responsible for an incorrect estimation of G0 and, consequently, a flawed interpretation of the viscosity decline.