Adsorption kinetics of the partially protonated cationic surfactant dodecylamine

The adsorption kinetics of the partially protonated cationic surfactant dodecylamine (C₁₂H₂₇N) in a 50 mM NaCl solution was examined. The dynamic and equilibrium surface tensions (STs) were measured using a video-enhanced pendant bubble tensiometer with a modified cell system. The equilibrium and complete ST relaxation profiles were compared with the theoretical profiles predicted from the non-ionic, partially ionized and fully ionized ionic models. A quasi-equilibrium approach was used in the ionic model to describe the electric field in the electrical double layer. In the very low surface pressure regime (π < 2 mN/m), the dynamic ST data were compared with the theoretical ST predicted by the ionic (partially or fully ionized) Frumkin model under a diffusion-controlled condition. The fitted theoretical profiles produced a diffusivity value of 0.6–1.0 × 10⁻⁶ cm²/s, which is much lower than the diffusivity calculated using the Wilke-Chang equation, which was D = 5.6 × 10⁻⁶ cm²/s. Therefore, it could be concluded that in the low surface pressure regime, the controlling mechanism of the adsorption process exhibits a mixed diffusive-kinetic control model.