Adsorption kinetics of the ionic surfactant decanoic acid

The adsorption kinetics of ionic surfactant decanoic acid was examined. Dynamic and equilibrium surface tensions (ST) were measured using a video-enhanced pendant bubble tensiometer. The equilibrium ST data and the complete ST relaxation profiles were compared with theoretical profiles predicted by both non-ionic and ionic models. The quasi-equilibrium approach was used in the ionic model to describe the electric field in the electrical double layer. Both non-ionic and ionic generalized-Frumkin models predict the equilibrium and dynamic ST data very well. Using the ionic model, a comparison between the dynamic ST data and the theoretical ST profiles gave a diffusivity of 5.5 × 10^-6 cm²/s. The adsorption of decanoic acid onto the air-water interface was determined to be diffusion controlled. Dynamic surface properties (Γ and ψ_s), the electrical potential ψ(r, t) in the double layer, and the double layer thickness λ(C, t) were also evaluated. It was found that the surface charge density increases very quickly at the early stages (∼1 s) of the surfactant adsorption process. Additionally, the electrical surface potential data implied that the surface charge density becomes significantly stronger at very dilute surfactant concentrations, where π is less than 1 mN/m.