Critical micelle concentration of mixed surfactant SDS/NP(EO)(40) and its role in emulsion polymerization

In emulsion polymerization of styrene, nearly all of the polymer particle nuclei are formed in micelles. Therefore, it is of fundamental importance to understand the relations involved in micelle formation. In this study, the critical micelle concentrations (CMC) of the mixed surfactant, sodium dodecyl sulfate (SDS)/nonylphenol tetracontylethoxylate (NP(EO)(40)), were determined for various compositions at 25 degrees C and at 80 degrees C by performing the surface tension measurements. The CMC data were well described by the regular solution model for mixed micelles. The system of mixed micelles exhibits a quite non-ideal behavior, especially at lower temperature (25 degrees C). The effect of the mixed surfactant SDS/NP(EO)(40) on the formation of latex particles was demonstrated by a series of styrene emulsion polymerization. Adding only a small amount of the anionic surfactant SDS into the polymerization system can dramatically increase the concentration of latex particles and also reduce the particle size of the latex product. Furthermore, emulsion polymerization of styrene stabilized by the mixed surfactant system does not follow the conventional Smith-Ewart theory when the level of NP(EO)(40) is relatively high.