The miscibility in binary mixtures of surfactants in the organized assemblies at the interface and in the solution is summarized on the basis of the phase diagrams of adsorption, micelle formation, and vesicle formation. In Section II, the thermodynamic equations to obtain the phase diagrams are given for some combinations of binary surfactant systems. Furthermore the criterion of an ideal mixing is proposed. In Section III, the miscibility in the adsorbed film and micelle is demonstrated for some representative systems by using the phase diagrams and the excess Gibbs free energy. In Section IV, the vesicle-micelle transition in the aqueous mixture and the phase transition in the adsorbed films at the oil/water interface are shown and examined from the viewpoint of the miscibility of the surfactants.
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