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Fractional counterion bindings were measured on micelles composed of mixtures of anionic/nonionic, cationic/nonionic, anionic/anionic, and cationic/cationic surfactants. Surfactants used were sodium decyl sulfate, sodium decyl sulfonate, hexadecylpyridinium chloride, hexadecyltrimethylammonium chloride, and a nonylphenol poly(ethoxylate). For ionic/nonionic mixed micelles, the fractional counterion binding varies monotonically between the value for the pure ionic micelle and zero, as the nonionic surfactant content of the micelle increases. For ionic/ionic mixed micelles, the binding varies monotonically between the values of the pure ionic micelles as composition varies. A localized adsorption model and a mobile adsorption model were developed to describe the binding. Both models were based on electrostatic considerations and both described the binding data very well. On physical grounds, the localized adsorption model is preferred, particularly for ionic/nonionic mixed micelles composed mainly of nonionic surfactant. These results indicate that electrostatic considerations are the dominant cause of the thermodynamic nonideality of the ionic/nonionic mixed micelle. Counterion bindings on mixed ionic/nonionic micelles vary little with temperature, a feature in common with micelles composed of a single ionic surfactant.

metal particles, the hydrogen attains the ZnO surface in an atomic form.Acknowledgment. The financial support of the Italian CNR "Progetto Finalizzato Material! e Dispositivi per F Elettronica a Stato Solido" is acknowledged.

The ultimate objective of the project was to investigate the relationship between microemulsion phase behavior and detergency for oily soils. In this study, surfactant phase behavior was evaluated for hexadecane and motor oil as model oily soils. Producing microemulsions with these oils is particularly challenging because of their large hydrophobic character. To produce the desired phase behavior we included three surfactants with a wide range of hydrophilic/lipophilic character: alkyl diphenyl oxide disulfonate (highly hydrophilic), dioctyl sodium sulfosuccinate (intermediate character), and sorbitan monooleate (highly hydrophobic). This mixed surfactant was able to bridge the hydrophilic/lipophilic gap between the water and the oil phases, producing microemulsions with substantial solubilization and ultralow interfacial tension. The effects of surfactant composition, temperature, and salinity on system performance were investigated. The transition of microemulsion phases could be observed for both systems with hexadecane and motor oil. In addition, the use of surfactant mixtures containing both anionic and nonionic surfactants leads to systems that are robust with respect to temperature compared to single-surfactant systems. Under conditions corresponding to "supersolubilization," the solubilization parameters and oil/microemulsion interfacial tensions are not substantially worse than at optimal condition for a middle-phase system, so a middle-phase microemulsion is not necessary to attain quite low interfacial tensions. A potential drawback of the formulations developed here is the fairly high salinity (e.g., 5 wt% NaCl) needed to attain optimal middle-phase systems. The correlation between interfacial tension and solubilization follows the trend predicted by the Chun-Huh equation.

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