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

Surface tension, heat capacity, and density of the sodium dodecyl sulf ate (NaDS)-dodecyldimethylamine oxide (DDAO) mixtures have been measur ed at 25-degrees-C as functions of the total molality. The critical mi celle concentration (cmc), derived from surface tension data, as a fun ction of the mixture composition (X(NaDS)) deviates negatively from th e ideal behavior. The apparent molar volumes (V(PHI,2)) and heat capac ities (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 m olality tending to a constant value. In case of C(PHI,2) the behavior is opposite. At a given mixture composition, the partial molar propert y of the mixed micelles (Y2,M) was evaluated on the basis of the pseud ophase transition model. This was done by extrapolating the partial mo lar property in the postmicellar region to the cmc. The V2,M as a func tion of the mixed micelle composition deviates negatively from the ide al behavior while Cp2,M shows a null deviation. These properties are d iscussed on the basis of the regular solution theory.