Effects of temperature, surfactant, and salt on the rheological behavior in semidilute aqueous systems of a nonionic cellulose ether

Oscillatory shear experiments have been carried out on thermoreversible gelling and nongelling semidilute aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) (at a constant polymer concentration of 1 wt %) in the presence of various amounts of sodium dodecyl sulfate (SDS) and at some different levels of NaCl addition. Depending on the concentrations of surfactant and salt, a temperature-induced sol-gel transition or only a viscosification of the solution was observed. For the gelling systems, the value of the gel temperature, determined by the observation of a frequency independent loss tangent, was found to be dependent on the composition of the system. At the gel temperature, a power law frequency dependence of the dynamic storage modulus (G' similar to omega(n') and loss modulus (G '' similar to omega(n '')) was constantly observed with n' = n '' = n. Values of the viscoelastic exponent n in the range 0.3-0.4 were reported. The value of n, as well as the gel strength parameter S, was dependent on the composition of the system. The rheological properties of the nongelling systems were affected by temperature, surfactant, and salt. In the absence of salt, the network structure is disrupted at high surfactant concentations and the dynamic viscosity decreases. However, if salt is added at this stage an enhanced viscoelastic response is observed and the network structure is re-established. The rheological results of this work indicate that the effects of surfactant and salt counteract each other. The present results for both gelling and nongelling systems are analyzed in a model where the interplay between swelling (caused by the ionic surfactant) and connectivity (established by ''lumps'' or hydrophobic associations) is considered.