Measurement of the Adsorption of Surface-Active Agent at a Solution - Air Interface by a Radiotracer Method

THE adsorption of a water-soluble surface-active agent at a solution - air interface has been measured by taking unique advantage of the soft beta radiation from the radioactive isotope of sulphur, S35. The method depends upon the fact that, because of 'self-absorption' of the radiation by the solution, the amount of radiation escaping from a solution of a compound containing this isotope comes principally from that fraction of the sulphur which is near the surface. Since 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 the activity from a solution (for example, of sodium sulphate) in which no surface adsorption occurs.