Rheology of hydrophobically modified polymers with spherical and rod-like surfactant micelles

Rheological measurements are used to probe the interactions between hydrophobically modified polymers and surfactant micelles. Hydrophobically modified hydroxyethylcellulose (hmHEC) with substitution levels of C₁₂ and C₁₆ hydrophobes between 0.9 and 7.3 mol% and hydrophobically modified polyacrylamide (hmPAM) with a sulfonated C₁₂ hydrophobe at 1 mol% show the effects of hydrophobe type and content on interactions. Sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and cetyldimethylamine oxide (CDMAO) surfactants show the effect of anionic, cationic, and nonionic surfactant interactions, respectively. Two patterns of association are observed: strong cooperative interaction and non-cooperative interaction. For the systems displaying cooperativity (e.g. SDS/hmHEC and CTAB/hmPAM), a large increase in viscosity is seen as a function of surfactant concentration, and the increase begins about a decade in concentration below the critical micelle concentration (cmc). For the non-cooperative interactions, the viscosity maximum occurs at concentrations higher than the cmc. For CTAB the addition of KBr and for CDMAO the addition of hexanol induce a sphere-to-rod transition in the surfactant phase. Whereas, the polymer-surfactant interaction is completely masked at high surfactant concentrations for spherical micelles, in the case of rod-like micelles high viscosity levels are maintained at high surfactant concentrations. Copyright (C) 1999 Elsevier Science B.V.