ABSTRACT
Precipitation of surfactants in aqueous solution is a phenomenon of great practical importance in such applications as detergency and petroleum production using surfactants. This chapter discusses both ionic surfactant precipitation by electrolytes and precipitation of anionic surfactants by cationic surfactants. Precipitation of anionic surfactants by added monovalent cations and divalent cations or by decreasing temperature is discussed. In all of these precipitation reactions, the presence of micelles decreases the tendency for precipitation to occur since precipitation can be viewed as a competition for
monomeric surfactant between micelles and precipitate. Thus, the tendency of anionic surfactant to precipitate is reduced when nonionic surfactant is added to the system. The concentration of cationic surfactant required to precipitate an anionic surfactant (or vice versa) is shown to increase above the critical micelle concentration of the system. Addition of nonionic surfactant to these anionic-cationic surfactant mixtures reduces the tendency for precipitation to occur. Precipitation rates are also discussed. The rate of precipitation is shown to be on the order of several minutes to reach equilibrium for pure anionic surfactant precipitated by calcium and on the order of 30min for anionic-cationic surfactant mixtures. In both cases, there is a period of inhibition during which little precipitation occurs. The more supersaturated a system is, the faster the precipitation reaches its equilibrium level. Mixtures of anionic surfactants can take substantially longer to precipitate than single surfactants.
