Surfactant Precipitation in Aqueous Solutions Containing Mixtures of Anionic and Nonionic Surfactants

The sal inity tolerance (precipitation phase boundary) is measured for a mixed anionic/nonionic surfactant sy s t em above the CMC. For any total surfactant concentration, the salinity tolerance is shown to increase as the percentage of nonionic surfactant in the s y s t e m is increased. A model is developed which can predict the phase houndaries for the mixed surfactant s y s t e m from the pure anionic surfactant phase boundary and information about mixed micelle formation. In the model, precipitation is v iewed as a solubil i ty product relationship between the anionic surfactant monomer and the total unassoc iated counterion. The reason that sal inity tolerance (or counterion concentration necessary to cause precipitation) increases with addition of nonionic surfactant is that mixed micelle format ion reduces the anionic surfactant monomer concentration. For the experimental studies, sodium dodecyl sul fate is the anionic surfactant , a polyethoxyl ated nonylphenol is the nonionic surfactant , and sodium chloride is the added salt.