Rheology and binding studies in aqueous systems of hydrophobically modified acrylamide and acrylic acid copolymers and surfactants

Viscosity data are reported for aqueous solutions of a series of acrylamide-based co- and terpolymers with added surfactants. Polymer composition is systematically varied through ionic content (acrylic acid monomer content and solution pH) and hydrophobicity (alkylacrylamide content and alkyl chainlength). Surfactants used were sodium dodecylsulfate (SDS), diethylhexyl sulfosuccinate (AOT), dodecyltrimethyl ammoniumbromide (DTAB) and nonionic surfactants. For the anionic surfactant SDS, a surfactant selective electrode was used to obtain binding isotherms of SDS with the polymers. The experiments show that in the presence of anionic surfactants, the solutions exhibit a dramatic increase in solution viscosity at concentrations around the surfactant CMC, attributed to interpolymer cross-linking through the formation of mixed micelles involving the hydrophobes from different polymer chains and the surfactant molecules. The viscosity enhancement is found to increase with increasing hydrophobicity of the hydrophobe and decreases with increasing AA incorporation in the polymer. The ionic fraction of the polymer chain (AA) also plays an important role in unfolding the polymer chain through electrostatic repulsion contributing to the viscosity increases at high solution pH. Both rheology and EMF-derived binding isotherms suggest that the viscosity maximum occurs at a low ratio of bound surfactant and hydrophobe monomers of approximately two surfactant molecules per hydrophobe.