By extending the thermodynamic method developed in this series, a thermodynamic formulation has been proposed to account for the behavior of the adsorbed films of binary surfactant mixtures at interfaces. Further, this formulation has been extended so as to be applicable to the mixed adsorbed films in equilibrium with mixed micelles. The surface tension of the aqueous solution of a decylammonium bromide (DeAB)-dodecylammonium chloride (DAC) mixture has been measured as a function of the total molality of DeAB and DAC and the mole fraction of DAC in the total surfactant at 298.15 K under atmospheric pressure. The total surface density and the compositions in the adsorbed film and micelle have been evaluated by applying the thermodynamic equations derived to the experimental results. It has been concluded on the basis of the surface tension-composition and critical micelle concentration (CMC)-composition diagrams that DeAB and DAC mix slightly nonideally with each other in both the adsorbed film and the micelle.
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