Adsorption of sodium dodecyl sulphate and propanol mixtures at aqueous solution–air interface

Surface tension measurements were carried out for aqueous solutions of sodium dodecyl sulphate and propanol mixtures. The obtained results were compared with those calculated from the relations derived by Joos, Miller and co-workers, and von Szyszkowski. From the comparison it appeared that using the modified adsorption isotherm derived by Joos the adsorption behavior of SDDS and propanol mixture can be predicted satisfactorily for each constant concentration of SDDS only at a low concentration of propanol; however, the values of the surface tension of aqueous solutions of SDDS and propanol mixtures can be predicted in the whole range of the propanol concentration studied using the modified von Szyszkowski equation. The values of the surface tension of the solutions of these mixtures can be also predicted by the simple relationships of Miller and co-workers for the description of the adsorption behavior of mixed surfactants systems, particularly at a low and high concentration of propanol in solutions. Next, using Gibbs and Szyszkowski equations, the surface excess of SDDS and propanol was determined. The obtained results indicate that there is a maximum of the excess concentration of SDDS and propanol in the mixed monolayer at aqueous solution–air interface at a low concentration of propanol and at SDDS concentration, which corresponds to the saturated monolayer of SDDS in the absence of propanol.