The surface tension of an aqueous solution of a dodecylammonium chloride (DAC) and octylsulfinylethanol (OSE) mixture was measured as a function of the total molality of surfactants and the mole fraction of OSE in the solution. By analyzing the experimental results using thermodynamic relations developed previously, a phase diagram of adsorption representing the equilibrium between the adsorbed film and solution was obtained at a given surface tension. At a low surface tension, the mixture was found to form a negative azeotropic mixed film. It was also observed that the phase diagram of micelle formation has a distinct minimum. Such behavior was attributed to the attractive interaction between DAC and OSE molecules in the oriented states. Further, the molecular interaction was found to be more attractive in the micellar state than in the adsorbed state.
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