A thermodynamic theory of microemulsions containing brine, oil, an ionic surfactant, and a nonionic cosurfactant is given. Conditions for phase equilibria are derived. The treatment is limited to droplettype microemulsions with emphasis on W/O + W systems. Important features are saturation adsorption of surfactant and cosurfactant, the interfacial bending stress, the standard chemical potential of the droplets in the free energy of mixing, and a quantitative treatment of the contribution of the electric double layer to the bending stress. Numerical illustrations show good agreement with experimental data on the influence of salt and cosurfactant on droplet size and interfacial tension in W/O + W equilibria
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