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

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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.
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