VOLUME AND HEAT-CAPACITY OF SODIUM DODECYL-SULFATE DODECYLDIMETHYLAMINE OXIDE MIXED MICELLES

Surface tension, heat capacity, and density of the sodium dodecyl sulfate (NaDS)-dodecyldimethylamine oxide (DDAO) mixtures have been measured at 25-degrees-C as functions of the total molality. The critical micelle concentration (cmc), derived from surface tension data, as a function of the mixture composition (X(NaDS)) deviates negatively from the ideal behavior. The apparent molar volumes (V(PHI,2)) and heat capacities (C(PHI,2)) Of the mixture were calculated from density and heat capacity data. At a given X(NaDS), V(PHI,2) increases with the total molality tending to a constant value. In case of C(PHI,2) the behavior is opposite. At a given mixture composition, the partial molar property of the mixed micelles (Y2,M) was evaluated on the basis of the pseudophase transition model. This was done by extrapolating the partial molar property in the postmicellar region to the cmc. The V2,M as a function of the mixed micelle composition deviates negatively from the ideal behavior while Cp2,M shows a null deviation. These properties are discussed on the basis of the regular solution theory.