The Influence of pH and Temperature on the Equilibrium and Dynamic Surface Tension of Aqueous Solutions of Sodium Oleate

Abstract

For aqueous solutions of sodium oleate, the dynamic surface tension (using the maximum bubble pressure technique) and the equilibrium surface tensions (using the du Noüy ring method) were studied. Experiments were carried out within the pH range 7 to 13 and at temperatures from 20 to 60°C. From the equilibrium surface tension studies at 25°C, a wide variation in surface activity was found to occur in the premicellar concentration range, depending on pH and oleate concentration, and this was explained on the basis of the formation of strongly surface-active premicellar species. It was also shown that the critical micelle concentration (cmc) of the system increased with pH within the pH range 7–12. Dynamic surface tension experiments were carried out from 20 to 60°C at concentrations beyond the cmc in alkaline solutions, and the adsorption kinetics at the air/solution interface were analyzed using established theoretical diffusion models. From these data, the effective diffusion coefficients (Deff) for the oleate species were determined and the presence of an interfacial activation barrier in the diffusion process was confirmed. It was found that the Deff values obtained within the temperature range from 40 to 60°C at 2–3 mM were in general agreement with previously reported values for the oleate dimer obtained by pulse-gradient FT-NMR. Furthermore, Deff significantly increased with temperature and also increased with concentration at higher temperatures (>40°C).