Polymer-surfactant interactions in dilute mixtures of a nonionic cellulose derivative and an anionic surfactant

The interaction between ethyl(hydroxyethyl)cellulose (EHEC) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in dilute aqueous solutions at different temperatures with the aid of viscometry. The results suggest that at low polymer concentrations the polymer-surfactant complexes are molecularly dispersed at the shear rates operating in the capillary viscometers used in this study, The delicate interplay between hydrophobic interactions and polyelectrolyte effects is demonstrated through the reduced viscosity data. At surfactant concentrations slightly above the critical aggregation concentration cac (the value of cac decreases with increasing temperature), the intrinsic viscosity features suggest a sharp collapse of the polymer-surfactant aggregates. At moderate amounts of surfactant, the molecular complexes expand due to amended thermodynamic conditions and enhanced electrostatic repulsion between chains decorated with SDS. At very high levels of surfactant addition, the small contraction of the molecular units is attributed to screening of the electrostatic interactions. At low surfactant concentrations (up to 4 mm), a temperature-induced shrinkage of the molecules is also observed. The very high values of the Huggins coefficient around cac suggest strong coil-coil interactions. The polymer-concentration-induced enhancement of the reduced viscosity around the cac is another indication of enhanced intermolecular interactions.