Effect of ethylene oxide addition number on the micro-regulation of fatty amine polyoxyethylene ether quaternary ammonium salt molecules and macroscopic foam properties.

The precise regulation of surfactant performance through molecular architecture is crucial for advanced colloidal applications. In this study, a homologous series of ethoxylated dodecyl-ammonium quaternary salts with varying ethylene oxide (EO) chain lengths (n = 2, 5, 10, 15) was synthesized to explore the non-linear effect of hydrophilic group size on physicochemical properties. Static surface tension and thermodynamic analysis showed a shift in the adsorption driving force from micellization-dominated (|ΔGmic| > |ΔGads|) at low EO numbers to adsorption-dominated (|ΔGads| > |ΔGmic|) at high EO numbers. Dynamic surface tension measurements revealed a critical kinetic transition: short-chain variants (S1202) followed diffusion-controlled adsorption, whereas long-chain counterparts (S1215) displayed significant kinetic hysteresis due to an energy barrier associated with interfacial conformational reorientation. Microscopic analysis (TEM/DLS) confirmed morphological changes from irregular lamellae/rods to spherical and eventually rod-like micelles, driven by a reduction in the critical packing parameter. Notably, the EO chain length served as a "molecular tuner" for macroscopic behavior: S1210 achieved the best balance in wetting dynamics, while S1202 exhibited superior foam stability due to the formation of a dense, rigid interfacial film that effectively suppressed Ostwald ripening and drainage. Additionally, a unique charge reversal (negative Zeta potential) was observed for S1210, attributed to the specific adsorption of anions on the Hydrophilic head group. This work establishes a comprehensive "structure-kinetic-performance" framework, offering theoretical guidance for the design of tailored surfactants.