Polymer-Induced Structural Transitions in Oleate Solutions:  Microscopy, Rheology, and Nuclear Magnetic Resonance Studies

Cryo-transmission electron microscopy (cryo-TEM), NMR, and rheometry were used to study polymer/surfactant interactions involving the nonionic polymers polyethylene glycol (PEG) and polypropylene glycol (PPG) and the surfactant potassium oleate in aqueous solution. In the absence of polymer, the surfactant solution was viscoelastic. Cryo-TEM showed that the surfactant formed threadlike micelles and NMR diffusion measurements also indicated a large micelle size. These micelles are similar to those formed by cationic alkyltrimethylammonium halides. Addition of PEG or PPG induced a viscoelastic-to-Newtonian liquid transition. In contrast to the behavior seen with the cationic surfactants, PEG is more effective than the more hydrophobic polymer, PPG, in inducing this transition. Cryo-TEM images showed a threadlike-to-spherical micelle transition in these polymer/surfactant systems. The extent of the threadlike-to-spherical transition detected by cryo-TEM for PEG compared to PPG correlates with the effectiveness of the two polymers in influencing solution rheology. Self-diffusion coefficients determined by NMR support an association between the surfactant and either polymer and a greatly reduced micellar hydrodynamic radius for the surfactant/polymer complex. The increase in surfactant diffusion coefficient caused by PEG compared to PPG also correlates with the effectiveness of the two polymers in influencing solution rheology. Two-dimensional nuclear Overhauser effect spectroscopy demonstrated a close proximity and long-lived interaction between oleate and PEG in the complex and suggests that all surfactant atoms have a measurable probability of being close to the polymer. The NMR data also established that internal aggregate dynamics are in the spin-diffusion (slow) limit.