Precipitation of mixtures of anionic and cationic surfactants: II. Effect of surfactant structure, temperature, and pH

Abstract

Precipitation phase boundaries for sodium alkyl sulfate/dodecylpyridium chloride were measured over a wide range of surfactant concentrations as a function of pH, temperature, and anionic surfactant alkyl chain length. Increasing temperature and decreasing surfactant alkyl chain length generally tend to decrease the tendency to precipitate. A previously developed model for predicting anionic/cationic surfactant precipitation phase boundaries described the experimental results very well except in some high surfactant concentration regions where coacervate and gels formed. This model uses a simple solubility product relationship, with regular solution theory, to describe mixed micelle formation. The environment of the alkyl chains is more favorable in the precipitate than in the micelles, from measured thermodynamic properties.