PRECIPITATION PHENOMENA IN MIXTURES OF ANIONIC AND CATIONIC SURFACTANTS IN AQUEOUS-SOLUTIONS

The precipitation phase boundary for mixtures of sodium dodecyl sulfate (NaDS) and dodecylpyridinium chloride (DPCl) is determined over a wide range of concentrations. The phase boundary is composed of a monomer-precipitate equilibrium curve, where no micelles exist in solution, and two branches (one NaDS-rich and one DPCl-rich) where monomer, micelles, and precipitate exist in equilibrium. A model is developed to predict the precipitation boundary by combining regular solution theory, to calculate monomer-micelle equilibrium, with a solubility product relationship between surfactant monomer concentrations, to calculate monomer-precipitate equilibrium. Results from the model are shown to work well, except in regions where coacervate formation occurs.