Surfactant precipitation in aqueous solutions containing mixtures of anionic and nonionic surfactants

The salinity tolerance (precipitation phase boundary) is measured for a mixed anionic/nonionic surfactant system above the CMC. For any total surfactant concentration, the salinity tolerance is shown to increase as the percentage of nonionic surfactant in the system is increased. A model is developed which can predict the phase boundaries for the mixed surfactant system from the pure anionic surfactant phase boundary and information about mixed micelle formation. In the model, precipitation is viewed as a solubility product relationship between the anionic surfactant monomer and the total unassociated counterion. The reason that salinity tolerance (or counterion concentration necessary to cause precipitation) increases with addition of nonionic surfactant is that mixed micelle formation reduces the anionic surfactant monomer concentration. For the experimental studies, sodium dodecyl sulfate is the anionic surfactant, a polyethoxylated nonylphenol is the nonionic surfactant, and sodium chloride is the added salt.