Measurement of the adsorption of surface-active agents at a solution/air interface by a radiotracer method

The adsorption of a water-soluble surface-active agent at the solution/air interface has been measured by taking advantage of the soft beta-radiation from the radioactive isotope of sulphur, 35 S. The method depends upon the fact that, because of 'self-absorption ’ of the radiation by the solution, and because the molecules of a surface-active agent are preferentially adsorbed at the interface, the radioactivity above a solution containing such an agent labelled with radiosulphur should be higher than that from a solution (e. g., of sodium sulphate) in which no surface adsorption occurs. Carefully purified di- n -octyl sodium sulphosuccinate and sodium sulphate were each synthesized with 35 S. Measurements on solutions of these showed that the radiotracer technique permitted a successful quantitative determination of the adsorption. Counting rates over the solutions of agent were 1.1 to 20 times those of the corresponding sodium sulphate, and for some 29 different solutions covering a range of specific activities (1 x 10 12 to 5 x 10 9 ) counts/min./mol.) and of concentrations (0.1 to 150 x 10 -8 mol./ml.) a well-defined adsorption isotherm resulted. The surface excess obtained from the tracer measurements agreed with that calculated from surface-tension measurements by the Gibbs equation if it was assumed that no Na + and only H + were associated with the long-chain ions adsorbed in the interface. The radioactivity measurements suggested further that, after the agent had formed a uni-molecular layer, a further increase in bulk concentration caused further adsorption at the interface. An interesting aspect is that the rate of adsorption was followed in the low concentration ranges. The method promises to be useful for studying the kinetics of such adsorption processes. The radiotracer method outlined above seems to be generally applicable to surface phenomena involving adsorbed layers, either mono- or multimolecular in nature. Although this paper describes the use of 35 S, it is evident that other radioisotopes having sufficiently soft radiation, such as 14 C, 45 Ca, 59 Fe, etc., could be employed. Moreover, the method appears to open considerable possibility for examining interaction in surface layers.