Mechanistic insight into multiscale self-assembly and viscoelastic enhancement in a CO₂-responsive amine/zwitterionic...

CO₂-responsive viscoelastic surfactant (VES) fluids are of growing interest for CO₂ flooding and fracturing in oil and gas development, yet the molecular mechanism by which CO₂-induced speciation, counterion environment, and hydrotrope-assisted association collectively regulate self-assembly and viscoelasticity in mixed amine/zwitterionic systems remains insufficiently resolved. Here, we use a previously reported amine/zwitterionic formulation composed of EKO, EAHSB and NaPts as a model system to clarify the structure-interaction-rheology relationship responsible for its non-additive viscoelastic enhancement. Rheological measurements show that the mixed EAHSB/EKO/NaPts-CO₂ system exhibits composition-dependent enhancement in zero-shear viscosity and elastic modulus, indicating cooperative supramolecular association rather than simple compositional averaging. Cryo-EM reveals the coexistence of vesicles, ring-like features, and thick cylindrical structures, pointing to a more diverse multiscale assembly landscape in the mixed system. ¹H NMR and coarse-grained molecular dynamics simulations further suggest that counterion environment and NaPts-involved interfacial reorganization alter the mixed headgroup region and are associated with richer multiscale assembly and enhanced viscoelastic response. Taken together, these results establish a mechanistic framework linking CO₂-induced protonation, NaPts-mediated mixed-headgroup organization, multiscale self-assembly, and macroscopic viscoelastic enhancement, thereby providing guidance for the rational design of CO₂-switchable amine/zwitterionic surfactant fluids.