Protonation-driven interfacial behavior and foam stability in mixed cationic–nonionic surfactant systems

Understanding how mixed ionic–nonionic surfactant systems control foam stability is critical for designing tailored interfacial systems in industrial and scientific applications. Here, we investigate the molecular mechanisms by which the cationic surfactant dodecylamine (DDA) and the nonionic surfactant methyl isobutyl carbinol (MIBC) regulate foam formation and stability. Using a multiscale approach combining macroscopic foam analysis (Dynamic Foam Analysis, DFA; Focused Beam Reflectance Measurement, FBRM), interfacial characterization (surface tension, zeta potential), and molecular-scale techniques ( 1 H Nuclear Magnetic Resonance, 1 H NMR and Small-Angle X-ray Scattering, SAXS), we link protonation-dependent behavior of DDA to interfacial adsorption, aggregation, and foam-stabilizing properties. At low DDA concentrations, protonated DDA (DDAH + ) forms uniform, positively charged interfacial layers, which synergize with MIBC to enhance interfacial packing and stabilize foam through combined electrostatic and hydration effects. At higher DDA concentrations, the increased fraction of neutral DDA disrupts interfacial charge uniformity and hydration layers, while MIBC promotes adsorption of unprotonated DDA, accelerating bubble coalescence and reducing foam stability. This integrated mechanistic study reveals how ionic surfactant protonation and co-adsorption with nonionic surfactants govern foam behavior, providing a framework for the rational design of mixed surfactant systems with controlled foam properties.