Viscosity of Aqueous Polyelectrolyte Solutions with Oppositely Charged Surfactant

The viscosity of polyelectrolyte solutions with oppositely charged surfactants is measured for a series of anionic polyelectrolytes of variable hydrophobicity (alternating copolymers of sodium maleate with hydrocarbon comonomers) in the presence of cationic trimethyl ammonium bromides with various alkyl tail lengths. These results are compared with a simple model that modifies the scaling theory for unentangled semidilute polyelectrolyte solutions to account for the addition of oppositely charged surfactant. The surfactant lowers the viscosity of these solutions through two means. The polyelectrolyte binds to the surface of the surfactant micelle, reducing the effective chain length of the polymer. The binding also causes counterions of the polyelectrolyte and the surfactant to be released into solution, acting as a salt that screens the repulsion between charges of the polyelectrolyte, causing the chains to have smaller size. The fraction of effectively charged monomers ($i.e$., free counterions) on the polyelectrolyte is measured via an ion-selective electrode, meaning the simple model has no adjustable parameters. Additional electrodes are used to measure the amount of free surfactant in solution in order to estimate the amount of surfactant associated with each polyelectrolyte chain.