Volumetric Mixing in Anionic/Nonionic, Cationic/Nonionic, and Anionic/Cationic Mixed Micelles

The volumetric mixing in anionic/nonionic, cationic/nonionic, and anionic/cationic mixed micelles was determined by examining the total surfactant apparent molar volumes (φ V,tot ) at total surfactant concentrations ( C T ) much greater than the mixture critical micelle concentration (CMC m ). The mixed surfactant systems investigated were sodium dodecyl sulfate and a polyethoxylated nonylphenol, at 0.15 M NaCl and with no added NaCl; cetyl pyridinium chloride and polyethoxylated nonylphenol, at 0.03 M NaCl; and sodium dodecyl sulfate and dodecyl pyridinium chloride, at 0.15 M NaCl. For all of the mixed surfactant systems investigated, the volumetric mixing in the mixed micelles at C T >> CMC m was ideal, even though the free energies of mixing at the onset of mixed micelle formation (as indicated by the mixture critical micelle concentrations at C T = CMC m ) exhibited strong negative deviations from ideality. Furthermore, added electrolyte was shown to have virtually no effect on the φ V,tot data (nor, consequently, the volumetric mixing) for the anionic/nonionic mixed surfactant system. The results of this study suggest that at C T >> CMC m , the electrostatic interactions do not significantly affect the molar volume of the mixed micelle. Therefore, the micelle hydrophobic core dominates the volumetric mixing in mixed micelles at C T >> CMC m .