Radiotracer Studies on Adsorption of Surface Active Substance at Aqueous Surface. III. The Effects of Salt on the Adsorption of Sodium Dodecylsulfate

Abstract

Abstract The adsorbed amounts were measured on the surface of aqueous solutions containing tritiated sodium dodecylsulfate (TSDS) and varying amounts of NaCl by the direct radiotracer method. The surface tensions were also measured for the same solutions by the drop-volume method. The Gibbs adsorption isotherm, derived without assuming the adsorption of the chloride ion, was applied, and the amounts of adsorption were calculated for both dodecylsulfate and chloride ions. The observed amounts of dodecylsulfate adsorption were confirmed to be in good agreement with those calculated, and the calculated amounts of chloride adsorption turned out to be nearly zero or slightly negative, which is usually theoretically assumed because of the ionic repulsion due to the adsorbed dodecylsulfate. Further, the adsorption of the dodecylsulfate ion attained a constant and saturate value beyond a certain concentration (SAC) below the critical micelle concentration (CMC). The logarithms of SAC were found to vary linearly with the logarithms of CMC for the range of salt concentration studied. The value of SAC being in a close relation with the surface phenomena is considered to be another measure of the surface activity of surfactants.