Demixing of Mixed Micelles. Thermodynamics of Sodium Perfluorooctanoate−Sodium Dodecanoate Mixtures in Water

Conductivity, density, heat capacity, enthalpy of dilution, and osmotic coefficient measurements of water−sodium perfluorooctanoate (NaPFO)−sodium dodecanoate systems were carried out as functions of the surfactants' total molality ( m t ) at different mole fractions ( X NaPFO ). From conductivity data, the critical micelle concentration (cmc) and the degree of ionization (β) of the micelles were derived. The cmc's of the micelles are higher than those of the pure surfactants while β depends linearly on X NaPFO . At a given mole fraction, the apparent molar volume ( V Φ ) and heat capacity ( C Φ ) of the mixture increases and decreases monotonically with m t, respectively. From data in the premicellar region, the standard (infinite dilution) partial molar properties ( Y o ) of the mixtures were calculated for both volume and heat capacity. Y o depends linearly on X NaPFO according to the ideal behavior. From data in the postmicellar region, the excess properties ( Y exc ) for the mixed micelles formation from pure micelles were calculated. The values of the excess volumes and heat capacities are positive and negative, respectively, in the whole range of composition. The excess free energy ( G exc ) is negative while both enthalpy and entropy 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 ≤ X NaPFO ≤ 0.6. According to this approach, G exc was calculated for some mixtures. It turned out that sodium dodecanoate and sodium dodecyl sulfate micelles are miscible in the whole range of composition while ammonium perfluorononanoate (NH 4 PFN) and ammonium dodecyl sulfate micelles are partially miscible, and the coexistence of the two mixed micelles pseudophases occurs in the range 0.43 ≤ X NH 4 PFN ≤ 0.75. In the case of the sodium decyl sulfate−sodium perfluorooctanoate system, only a critical point is present.