Radiotracer Studies on Adsorption of Surface Active Substance at Aqueous Surface. IV. Direct Measurement of Adsorption of Tritiated Nonionic Surfactant

The radio- and surface-chemically pure nonionic surfactant, C10H21(CHT)2O(C2H4O)6H, was synthesized, and surface tension and adsorbed amount of its aqueous solution were measured directly at the air-solution interface at 30°C. Surface tension was measured by the drop-volume and Wilhelmy plate methods, while adsorbed amount was measured by the radiotracer method. The adsorption isotherm showed saturation values of 2.73×10−10 mol/cm2 above the concentration of about 1/5 of the critical micelle concentration. The observed amount of adsorption was in good agreement with the values calculated from surface tension data measured by the Wilhelmy plate method by applying the Gibbs adsorption isotherm for a single solute system. The time dependence of surface tension of surfactant solution was further discussed, and the inherent time dependence was shown for pure surfactant solution.