The adsorption at solution–air interface and volumetric properties of mixtures of cationic and nonionic surfactants

Surface tension, density and conductivity measurements were carried out for systems containing mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol), Triton X-100 (TX100). The obtained results of the surface tension measurements were compared with those calculated from the relations derived by Joos, Miller and co-workers. From the comparison it appeared that using the modified adsorption isotherm derived by Joos the adsorption behaviour of CTAB and TX100 mixture can be predicted satisfactorily; however, the values of the surface tension of aqueous solutions of these mixtures calculated on the basis of the simple relationships of Miller et al. are a little higher than those measured in contrast to many two component systems of surfactants studied earlier. On the basis of the results obtained from the measurements and calculations it was found that there is a linear relationship between the surface tension and composition of CTAB and TX100 mixtures at their low concentration; however, in the concentration range corresponding to that of the saturated monolayer at the interface a negative deviation from the linear relationship is observed. This fact and the values of the parameters of molecular interactions in the mixed monolayer suggest that there is synergism in the reduction of the surface tension of aqueous solutions of CTAB and TX100 mixture when saturation of the monolayer is achieved. The negative parameters of intermolecular interaction in the mixed micelle and calculations based on MT theory of Blankschtein indicate that there is also synergism in the micelle formation for CTAB and TX100 mixture. It was also found that the values of the standard free energy of micellization for the mixture of CTAB and TX100 are somewhat lower than for individual components and they can be predicted on the basis of Maeda equation and the mole fraction of surfactant and free enthalpy of mixing CTAB and TX100 in the micelle. Our measurements and calculations indicate that the volume both of the individual surfactants and mixture studied decreases during the micellization process; however, at the concentration of the surfactants higher than CMC an increase of this volume is observed.