Synergistic modulation of interfacial kinetics and self-assembly in dodecyl fatty amine polyoxyethylene ethers: the pivotal role of counterion charge density and EO chain length

Abstract

The regulation of interfacial phenomena represents an enduring Frontier in modern condensed matter physics and chemical engineering. Systematic thermodynamic and kinetic evaluations, utilizing dynamic light scattering and equilibrium surface measurements, elucidate the regulatory mechanisms of counterion species (Cl⁻ vs. CH₃SO₄⁻) and polyoxyethylene (EO) chain lengths on dodecyl fatty amine polyoxyethylene ether quaternary ammonium salts. Methosulfate (CH₃SO₄⁻) series consistently exhibit superior wetting and foaming efficiencies compared to chloride-based systems. This enhanced performance arises from a lower charge density that effectively screens the Stern layer and diminishes the adsorption energy barrier. A non-monotonic structural dependence on EO chain length characterizes the system; extended chains (EO = 15) impart robust steric stabilization yet induce a macroscopic “wetting inertia” due to elevated entropic barriers at the triple-phase contact line. Furthermore, a pronounced reversal in antistatic efficacy is observed: dimethyl sulfate-based systems deliver superior performance for short-chain structures (EO = 2), whereas chloride-based systems dominate at higher ethoxylation degrees (EO = 15). Thermal analysis indicates that bulky CH₃SO₄⁻ ions enhance molecular stability via charge delocalization, whereas Cl⁻ ions induce early thermal degradation by promoting Hofmann elimination. Finally, emulsification stability in polar oils is governed by dipole–dipole interactions, in contrast to the steric hindrance mechanisms prevalent in non-polar systems. These findings establish definitive, molecular-level design rules for optimizing cationic surfactants in advanced industrial formulations.