was not reported (since these studies examined only one The volumetric mixing in anionic/nonionic, cationic/nonionic, mixed surfactant composition). Total surfactant apparent and anionic/cationic mixed micelles was determined by examining molar volumes have been successfully used to determine the total surfactant apparent molar volumes ( f V,tot ) at total surfacthe volumetric mixing in nonionic/nonionic (3), cationic/ tant concentrations (C T ) much greater than the mixture critical cationic (4), and cationic/nonionic (4) mixed micelles. Surmicelle concentration (CMC m ). The mixed surfactant systems inprisingly, all of these mixed surfactant systems were shown vestigated were sodium dodecyl sulfate and a polyethoxylated nonto have a zero excess volume of mixing in the mixed micelles ylphenol, at 0.15 M NaCl and with no added NaCl; cetyl pyridin-(3, 4); therefore, the volumetric mixing in these nonionic/ ium chloride and polyethoxylated nonylphenol, at 0.03 M NaCl; nonionic, cationic/cationic, and cationic/nonionic mixed and sodium dodecyl sulfate and dodecyl pyridinium chloride, at 0.15 M NaCl. For all of the mixed surfactant systems investigated, micelles was ideal. the volumetric mixing in the mixed micelles at C T Σ· CMC m was
The main objective of this study was to further investigate ideal, even though the free energies of mixing at the onset of mixed the effects of the electrostatic interactions on the volumetric micelle formation (as indicated by the mixture critical micelle mixing in mixed micelles and then to compare these results concentrations at C T Γ
CMC m ) exhibited strong negative deviawith the pseudo-phase separation modeling of the mixture tions from ideality. Furthermore, added electrolyte was shown to critical micelle concentrations (when available), to obtain have virtually no effect on the f V,tot data (nor, consequently, the a more complete picture of the thermodynamics of mixed volumetric mixing) for the anionic/nonionic mixed surfactant sysmicelle formation. Several binary mixed surfactant systems tem. The results of this study suggest that at C T Σ· CMC m , the were investigated, including anionic/nonionic surfactant electrostatic interactions do not significantly affect the molar volmixtures with and without the presence of added electrolyte, ume of the mixed micelle. Therefore, the micelle hydrophobic core cationic/nonionic surfactant mixtures, and anionic/cationic dominates the volumetric mixing in mixed micelles at C T Σ· CMC m . α§ 1997 Academic Press surfactant mixtures. To the best of our knowledge, the volumetric mixing in anionic/nonionic and anionic/cationic mixed micelles has not been previously reported, nor has purification procedure may be found elsewhere (5).