The properties of mixtures of two anionic surfactants in water at the water ∣ air interface

Surface tension measurements were carried out for systems containing mixtures of sodium dodecyl sulphate and sodium hexadecyl sulphonate. From the results of these measurements the mole fraction of sodium hexadecyl sulphonate in a mixed monolayer and in mixed micelles, and the molecular parameters for the formation of mixed micelles and mixed monolayers at water ∣ air interface were determined. Also, the values of the surface tension of the studied systems have been calculated using the relation derived by Joos [Bull. Soc. Chim. Belg., 76 (1967) 591] and compared with the measured values. From this comparison, the adsorption behavior of SDDS and SHDSs mixture can be predicted by the Joos relation, particularly at low surface pressure. The calculated parameters indicate that in the systems of two anionic surfactants no synergism is present in adsorbed monolayers and in the micelle formation, however, a small maximum of the surface concentration excess at water ∣ air interface and a minimum of surface tension corresponding to CMC at α=0.4 and α=0.2 was, respectively, observed.