INVESTIGATION OF MIXING IN BINARY SURFACTANT SOLUTIONS BY SURFACE-TENSION AND NEUTRON REFLECTION - STRONGLY INTERACTING ANIONIC ZWITTERIONIC MIXTURES/

Aqueous solutions of the strongly interacting anionic/zwitterionic sur factant mixture of sodium dodecyl sulfate (SDS) and dodecyl dimethylam ino acetate (C(12)betaine) have been studied by means of surface tensi on and neutron reflection. The mixed critical micelle concentrations ( cmc) were used to derive the interaction parameter PM for micellizatio n, which was found to be large and negative. It was also found to be c omposition dependent and therefore not to obey the first-order approxi mation for the activity coefficients in the pseudophase separation app roximation. The temperature dependence of the mixed smc gave the therm odynamic excess functions for micellization; S-E was found to be posit ive over most of the composition range. Application of the pseudo-phas e separation model to surface tension data was used to show that the i nteraction parameter beta(sigma) in the surface layer is similarly lar ge and negative. However, direct measurement of the surface concentrat ions using neutron reflection disagree with the predictions of the pse udo-phase separation model and indicate that beta(sigma), although neg ative, is much smaller in magnitude. The structure of the mixed layer was determined at three compositions and found to be significantly deh ydrated in comparison with layers of the single surfactants, which may explain the positive excess entropies observed for both micellization and surface mixing. It is also suggested that changes of hydration on mixing invalidate the use of the pseudo-phase separation model and ma y be responsible for the deviations from the first-order model observe d for both micellization and surface interaction.