Radiotracer Studies on Adsorption of Surface Active Substance at Aqueous Surface. II. The Effect of Excess Salt on the Adsorption of Sodium Dodecylsulfate

Abstract

Abstract The adsorbed amount and the surface tension at the air-solution interface of the solution of tritiated sodium dodecylsulfate in the presence of excess common inorganic salt have been accurately measured at 25°C. The radioactivity of tritium β-rays over the solution was counted with a sheet scintillation counter and the surface tension of the solution was measured by a drop volume method. In the sodium dodecylsulfate solution containing sodium chloride of 0.115 mol/l, the adsorbed amount of dodecylsulfate ions was saturated to give 4.33×10−10 mol/cm2 when the bulk concentration was higher than 5.0×10−4 mol/1000 g solution. In the presence of the excess inorganic salt common to the ionic surfactant in solution the Gibbs adsorption isotherm has been experimentally proved to be valid for n=1. Moreover, it was confirmed that the equation of state for the ionic adsorbed film on the salt-free solution proposed previously is also applicable to the ionic surfactant solution with the excess salt.