Rheology and interaction mechanisms of polyelectrolyte-wormlike micellar surfactant complexes: A two-dimensional correlation spectroscopy and time-concentration superposition study

This study investigates the rheological behavior and molecular interaction mechanism of polyelectrolyte–wormlike micellar complexes formed by potassium oleate (KOL) with either ε-poly-L-lysine (PL) or poly(acrylic acid) (PAA) in water. Steady shear and oscillatory rheology experiments, combined with confocal laser scanning microscopy (CLSM), were performed to evaluate the effect of polyelectrolyte charge and concentration on the micellar organization. In KOL–PL systems, a critical PL concentration (≈1.5%) triggered a sharp viscosity increase and complex flow behavior, associated with the formation of supramolecular or coacervate-like aggregates. CLSM confirmed the appearance of structured domains at high PL content. By contrast, KOL–PAA mixtures exhibited a progressive increase in viscosity and viscoelastic moduli upon PAA addition although no aggregate could be detected by visual inspection, suggesting the onset of rearrangements at the nanoscale. Two-dimensional correlation spectroscopy (2D-COS) of attenuated total reflection Fourier-transform infrared (ATR-FTIR) data revealed distinct interaction pathways: in KOL–PL the carboxylate headgroups are the most sensitive moieties to polyelectrolyte addition, followed by alkyl tails and hydration water, consistently with a direct electrostatic binding. In KOL–PAA, the first response occurred in the hydration layer and hydrogen-bonding environment before reaching the micellar headgroups and cores, suggesting an interaction mechanism mediated by counterions and hydrogen bonds. Finally, a time–cosolute concentration superposition (TCCS) approach was successfully applied to the rheological data of both systems to obtain unified master curves and scaling laws, demonstrating its predictive value for polyelectrolyte–micelle complexes.