The properties of mixtures of two cationic surfactants in water at water/air interface

Surface tension and conductivity measurements were carried out for systems containing mixtures of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPyB). 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 behavior of CTAB and CPyB mixture can be predicted satisfactorily, particularly at a low concentration of the mixture; however, the values of the surface tension of aqueous solutions of these mixtures can be predicted in the whole range of the mixture concentrations by the simple relationships of Miller et al. On the basis of the results obtained from measurements and calculations it was found that there is a linear relationship between the surface tension and composition of two cationic surfactant mixtures at their low concentration, however, at a concentration close to the critical micelle concentration (CMC) positive deviation from the linear relationship is observed. There can also occur positive deviation from linear relationships between CMC and the degree of surfactants ionisation in micelles and mole fraction of CTAB (α) in mixture. In the case of excess concentration of CTAB and CPyB mixtures it was found that the maximal concentration of the mixture was somewhat higher than that for individual surfactants in the saturated monolayer. It was concluded that maximum adsorption probably results from changes of structure of the CPyB molecules in the mixed monolayer in comparison to their structure in the bulk phase and in the saturated monolayer of the individual surfactants.