Phase equilibrium in aqueous mixtures of nonionic and anionic surfactants above the cloud point

Abstract

The phase equilibrium between the two isotropic phases formed above the cloud point in an aqueous solution containing anionic surfactant, nonionic surfactant, and salt was studied as a function of the anionic/total surfactant ratio in the concentrated or coacervate phase under isothermal conditions. As the anionic/total surfactant ratio increased, the system approached the lower consolute solution temperature and the two phases became increasingly similar in composition. The concentration of surfactant in the dilute phase was found to be well above the CMC. The activity coefficient of each component was determined in the coacervate phase. The activity coefficients of the surfactant components in the coacervate were also computed on a surfactant-only basis. There is substantial negative deviation from ideality of mixing between the dissimilar surfactants in the coacervate. Regular solution theory provides an approximate description of the anionic/nonionic surfactant interaction in the coacervate. A thermodynamic consistency test found the true activity coefficients in the coacervate to be internally consistent, but the surfactant-only based activity coefficients failed the test, bringing into question the validity of using the pseudo-phase separation model to describe surfactant interactions in the coacervate.