The surface tension of the aqueous solution of octyldimethylphosphine oxide (OPO) and decyldimethylphosphine oxide (DePO) mixture was measured as a function of the total molality and the composition of the mixture at 298.15 K under atmospheric pressure. The experimental data were analyzed thermodynamically to evaluate the total surface density ΓH, the composition in the adsorbed film X2H, and the composition in the micelle X2M. The relations among the compositions of bulk solution, adsorbed film, and micelle were expressed in the form of phase diagram similar to the three-dimensional one; they were compared with the phase diagrams of ideal mixed film and micelle obtained theoretically. It was concluded that the OPO and DePO molecules mix ideally in the adsorbed film while the difference in their hydrophobic chain length causes a weaker interaction in the micelle because of a spherical shape of the micelle. This conclusion was supported by the fact that the 2-(octylsulfinyl)ethanol and 2-(decylsulfinyl)ethanol mixture behaves quite analogously.

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The miscibility of dodecyltrimethylammonium chloride (DTAC) and dodecylammonium chloride (DAC) in the adsorbed film at water/air interface and in the micelle was investigated by measuring the surface tension of the aqueous solution of surfactant mixture as a function of the total molality and composition of surfactants. The compositions of surfactants in the adsorbed film and micelle were evaluated by use of the thermodynamic equations developed previously. The results indicated that DTAC and DAC molecules mix slightly nonideally in the adsorbed film and micelle. Furthermore, the composition of DAC in the micelle was found to be remarkably smaller than that in the adsorbed film at the critical micelle concentration. It was concluded that the bigger hydrophilic group of DTAC is geometrically favorable for the micelle formation.