The pH regulated surface activity of alkanoic acids

We applied the model of adsorption of ionic and non-ionic surfactant mixtures, being an extension of previously developed model of adsorption of ionic surfactants, for the description of the pH regulated surface activity of alkanoic acids. We determined the surface tension isotherms for aqueous solutions of octanoic acids and its sodium salt – sodium octanoate, with various additions of NaOH and HCl. A very peculiar shape of isotherms, which was the result of acid–base equilibrium and a large difference of surface activity of octanoate ions and neutral octanoic acid molecules, could be well described by the theoretical model. Additional measurements of surface potential isotherms were correlated with the surface tension and could also be described by our theoretical model.