Measurement of Adsorbed Amount of Sodium Dodecylsulfate at Gas-Solution Interface by Radiotracer Method Using Tritium-Labeled Compounds

Abstract

Abstract Sodium dodecyl-2,3-3H,3Hsulfate (SDS) was synthesized from non-radioactive dodecanol, and the adsorbed amount of SDS at the gas-water interface, Γ, was directly measured by the radiotracer method with the use of 2π gas-flow windowless counter and humidified propane as a flow-gas. Because of a very soft nature of tritium β-rays, surface radioactivity is exclusively due to the solute adsorption compared with the large contribution from the bulk of the solution as in the case of 14C or 35S labeled detergents. Therefore the use of 3H labeled compounds enables the determination of the amount of adsorption in high accuracy, even in the region of high concentration beyond the critical micelle concentration (CMC) of SDS solution. However, no reliable reports have appeared as yet on the studies of adsorption of surfactant at gas-solution interface using 3H labeled compounds. Our measurement was carried out under exactly specified conditions, namely using chemically and radiochemically pure sample and under the condition free from evaporation. It was confirmed that Γ increased with the increase of SDS concentration and approached the saturation value of 5.9×10−10 mol/cm2 or 28.2 Å2/molecule at the concentration higher than 6×10−3 mol/1000 g solution and beyond CMC. Furthermore, Γ proved to be independent of the acidity of solution above pH 4, while it increased with increasing acidity below pH 4. The phenomenon is explained by the presence of hydrolytically formed dodecylsulfuric acid whose surface activity is higher than that of SDS.