PRECIPITATION OF MIXTURES OF ANIONIC AND CATIONIC SURFACTANTS .3. EFFECT OF ADDED NONIONIC SURFACTANT

The precipitation of an anionic surfactant by a cationic surfactant in the presence of a nonionic surfactant is examined. The precipitation domains for sodium dodecyl sulfate/dodecylpyridinium chloride were mea sured over a wide range of surfactant concentrations as a function of nonylphenol polyethoxylate concentration. Increasing the nonylphenol p olyethoxylate concentration decreases the tendency for precipitation t o occur. A model for predicting precipitation domains in ternary surfa ctant mixtures has been developed and verified experimentally. The mod el allows the nonionic surfactant to affect the precipitation behavior only by lowering the critical micelle concentration of the mixture. S mall deviations between theory and experiments along part of the anion ic-rich micelle boundary result from adsorption of SDS on the precipit ate which gives the microcrystals a negative charge and prevents their growth to a visible size. (C) 1994 Academic Press, Inc.