DEMIXING OF MIXED MICELLES - THERMODYNAMICS OF SODIUM PERFLUOROOCTANOATE SODIUM DODECANOATE MIXTURES IN WATER

Conductivity, density, heat capacity, enthalpy of dilution, and osmoti c coefficient measurements of water-sodium perfluorooctanoate (NaPFO)- sodium dodecanoate systems were carried out as functions of the surfac tants' total molality (m(t)) at different mole fractions (X(NaPFO)) Fr om conductivity data, the critical micelle concentration (cmc) and the degree of ionization (beta) of the micelles mere derived. The cmc's o f the micelles are higher than those of the pure surfactants while bet a depends linearly on X(NaPFO). At a given mole fr action, the: appare nt molar volume (V-Phi) and heat capacity (C-Phi) of the mixture incre ases and decreases monotonically with m(t), respectively. From data in the premicellar region, the standard (infinite dilution) partial mola r properties (Y-o) of the mixtures were calculated for both volume and heat capacity. Y-o depends linearly on X(NaPFO) according to the idea l behavior. From data in the postmicellar region, the excess propertie s (Y-exc) for the mixed micelles formation from pure micelles were cal culated. The values of the excess volumes and heat capacities are posi tive and negative, respectively, in the whole range of composition. Th e excess free energy (G(exc)) is negative while both enthalpy and entr opy are positive, On the basis of the pseudophase transition model and experimental evidences, the G(exc) vs composition profile was derived , This profile indicates the presence of a critical point for 0.4 less than or equal to X(NaPFO) less than or equal to 0.6. According to thi s approach, G(exc) was calculated for some mixtures. It turned out tha t sodium dodecanoate and sodium dodecyl sulfate micelles are miscible in the whole range of composition while ammonium perfluorononanoate (N H4PFN) and ammonium dodecyl sulfate micelles are partially miscible, a nd the coexistence of the two mixed micelles pseudophases occurs in th e range 0.43 less than or equal to X(NH4PFN) less than or equal to 0.7 5. In the case of the sodium decyl sulfate-sodium perfluorooctanoate s ystem, only a critical point is present.