A Study of the Adsorption of Detergents at a Solution-Air Interface by the Radiotracer Method. V. The Effect of Inorganic Salts on the Surface Behavior of the Dodecyl Sulfate Ion

Abstract The adsorption of 35S-labeled dodecyl sulfate ion in the presence of excess sodium salts with anions of various valencies was measured directly by the radiotracer method. The anions of the excess salts were chloride, nitrate, carbonate, ferricyanide and ferrocyanide ions, and the concentrations of the excess salts were all 0.1 n. It was found that the adsorption of dodecyl sulfate ion increased remarkably due to the presence of the excess salt and that the surface tension curves and the adsorption isotherms were almost independent of the type of anions. The behavior of dodecyl sulfate ion was discussed by using the result of the present study together with the previous one, in which the effect of cationic valency and the salt concentration was studied. The logarithmic value of CMC was found to be linear against the logarithmic concentration of excess sodiumchloride and against the valency of cation when the concentrations were all 0.1 n. The reduced surface pressure and the reduced concentration referred to CMC were proposed, and it was found that the surface tension data of the solutions of sodium dodecyl sulfate containing various kinds of salts and varying concentration of sodium salts were correlated approximately to one curve regardless both of the types and the concentrations of the excess salts. The curve was well expressed by the reduced equation for the surface pressure as a function of the reduced concentration. By combining the reduced equation with the Gibbs equation of n=1, the reduced adsorption isotherm was derived. It was shown that these reduced equations enable us to reproduce the surface tension and adsorption data of the solutions of sodium dodecyl sulfate in the presence of sodium salts with varying valencies of anions.