functions of both the total concentration and the mixture Conductivity, density, heat capacity, enthalpy of dilution, and composition. To the best of our knowledge, there are sparse osmotic coefficient measurements of water-sodium dodecyl suldata concerning partial molar volumes (7-9) and enthalpies fate (NaDS) -sodium dodecanoate systems were carried out as of mixing . A systematic thermodynamic investigafunctions of the surfactants total molality (m t ) at different mole tion is available only for dodecyldimethylamine oxide-sofractions (X NaDS ). From conductivity data, the critical micelle condium dodecyl sulfate mixture for which partial molar volcentration (cmc) and the degree of ionization of the micelles ( b) umes, heat capacities, enthalpies, free energies, and entropies were calculated. The cmc vs X NaDS profile deviates negatively from are reported in the literature (12-13). Complete sets of that predicted on the basis of the Clint approach, while the b vs thermodynamic data are of great importance since they can X NaDS profile deviates positively from the line correlating the values for pure surfactants. At a given mole fraction, the profiles of all be used to test theories and models and to obtain information the investigated properties as functions of m t qualitatively do not on the interaction governing the mixed micelles formation. differ from those for pure surfactants and lie between those for The thermodynamics of this process is generally discussed the two pure surfactants. By using a previously reported thermodyin terms of excess properties, i.e., the difference between namic approach, from the apparent molar properties in the postthe partial molar properties of the mixed micelles and those micellar region the excess properties for the mixed micelles formacalculated according to the ideal behavior, as functions of tion were evaluated. The excess volumes and entropies are positive, the mixture composition. The results are usually compared while the excess heat capacities, enthalpies, and free energies are with those predicted from models based on the regular solunegative in the whole range of the mixtures composition. The tion theory (14) which, developed for hydrocarbons mixabsolute values of the excess properties are generally small with tures, assumes the ideal behavior for volume and entropy of respect to those for other previously investigated systems, i.e.,
mixing. This hypothesis is not verified for the mixed micelles sodium dodecyl sulfate-dodecyldimethylamine oxide and sodium decylsulfate-dodecyldimethylamine oxide.