Adsorption kinetics of the partially dissociated anionic surfactant Aerosol-OT in 10 mM NaCl solution

The adsorption kinetics of anionic surfactant Aerosol-OT (AOT) in 10 mM NaCl solution was investigated by measuring and simulating the dynamic and equilibrium surface tension (ST) data. The relaxation of ST for AOT molecules adsorbing onto a freshly created bubble surface was measured using a video-enhanced pendant bubble tensiometer at eighteen AOT concentrations. The partially ionized Langmuir model was applied to best-fit the dynamic/equilibrium ST data. The subsequent findings revealed that the model with a dissociation degree α of 0.1 − 0.2 described the dynamic and equilibrium ST data very well. This implied that AOT behaved as a partially ionized surfactant in 10 mM NaCl solution. The adsorption for AOT onto a clean air–water interface was found to be either (i) diffusion-controlled with a diffusivity value that varied [between 2.5 and 4.6 (10⁻⁶ cm²/s)] depending on the bulk concentration or (ii) mixed-controlled with an adsorption rate constant of 2.4 ± 0.8 (10⁷ cm³/mol.s).