The surface tension of the aqueous solution of decylammonium bromide (DeAB) and decylammonium chloride (DeAC) mixture was measured as a function of the total molality and composition of surfactants at 298.15 K under atmospheric pressure. By applying the thermodynamic equations derived previously to the experimental results, the compositions in the mixed adsorbed film and mixed micelle were evaluated. It was found that the adsorbed film and micelle are rich in DeAB, which is more surface active than DeAC, compared with the aqueous solution. Further, the DeAB and DeAC molecules were concluded to mix ideally in the states of the adsorbed film and micelle. On the other hand, the compositions were compared with those of dodecylammonium chloride (DAC)-DeAC mixture at the critical micelle concentration (one). The discrep-ancy between the compositions was proved to be related closely to the differences between the erne's and between the surface tension values at the cmc of pure surfactants.
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