Influence of binary surfactant mixtures on the rheology of associative polymer solutions

Hydrophobically modified alkali-soluble emulsion polymers (HASE) are a class of comblike associative polymers that can impart high viscosities to aqueous solutions. The rheology of HASE solutions can be tuned by the addition of surfactants, such as nonylphenol ethoxylates (NP e), where e is the length of the hydrophilic (ethoxylate) chain. While previous studies have considered individual surfactants, our focus here is on binary surfactant mixtures. We find that equimolar NP4-NP12 mixtures significantly enhance the zero-shear viscosities of HASE solutions as compared to equivalent amounts of NP8, especially at high overall surfactant concentrations. Dynamic rheological measurements suggest that the higher viscosities are due to increases in the lifetime of hydrophobic junctions in the polymer-surfactant network. In contrast to the above results, equimolar NP4-NP8 mixtures are rheologically identical to equivalent solutions of NP6. The differences between the two sets of mixtures are further correlated with cloud point measurements and thereby with the overall hydrophilic-lipophilic balance (HLB) of the surfactant system.