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RESEARCH PRODUCT

Thermodynamic Studies of Sodium Dodecyl Sulfate–Sodium Dodecanoate Mixtures in Water

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Abstract Conductivity, density, heat capacity, enthalpy of dilution, and osmotic coefficient measurements of water–sodium dodecyl sulfate (NaDS)–sodium dodecanoate systems were carried out as functions of the surfactants total molality ( m t ) at different mole fractions ( X NaDS ). From conductivity data, the critical micelle concentration (cmc) and the degree of ionization of the micelles (β) were calculated. The cmc vs X NaDS profile deviates negatively from that predicted on the basis of the Clint approach, while the β vs X NaDS profile deviates positively from the line correlating the values for pure surfactants. At a given mole fraction, the profiles of all the investigated properties as functions of m t qualitatively do not differ from those for pure surfactants and lie between those for the two pure surfactants. By using a previously reported thermodynamic approach, from the apparent molar properties in the postmicellar region the excess properties for the mixed micelles formation were evaluated. The excess volumes and entropies are positive, while the excess heat capacities, enthalpies, and free energies are negative in the whole range of the mixtures composition. The absolute values of the excess properties are generally small with respect to those for other previously investigated systems, i.e., sodium dodecyl sulfate–dodecyldimethylamine oxide and sodium decylsulfate–dodecyldimethylamine oxide.