The anomalous behavior of surface tension of aqueous solutions of dimethyloleylamine oxide, and its multimolecular adsorption on aqueous surfaces

Abstract

Surface tension of aqueous solutions of dimethyloleylamine oxide in the absence of HCl and in the presence of 10 −4, 10 −3 and 10 −2 M HCl has been measured by the drop weight method over the surfactant concentration range 0–10 −3 M. The surface tension drops suddenly above 10 −5 M and becomes constant above 10 −4 M, in most cases. It does not show any sign of electrolyte adsorption on aqueous surfaces, but it has a similar dependence on surfactant concentration, irrespective of the addition of HCl. The application of the Gibbs adsorption isotherm, together with the assumption of solution ideality, leads to the surface excess density of total surfactant, suggesting the formation of multimolecular layers. On water and 10 −4 M HCl, the adsorbed surfactants form quadrimolecular layers but, on 10 −3 and 10 −2 M HCl, bimolecular layers are formed. In the presence of HCl, especially to 10 −4 M, the surface excess density of the surfactant has a maximum at a certain surfactant concentration. The occurrence of a maximum in the adsorption may be explained qualitatively by assuming that only the concentration of monomeric surfactant is directly related to the adsorption and that partially ionized micelles are formed.