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This paper deals with theoretical investigation of interfacial properties between two coexisting liquid or fluid phases in thermodynamical equilibrium. The Cahn-Hilliard gradient theory was combined with an activity coefficient model (Koningsveld-Kleintjens model) or with an equation of state (Sanchez-Lacombe lattice fluid model). Using an activity coefficient model, only one variable (concentration) changed passing the interface. The interfacial tension between demixed liquids was calculated for two systems (water + ethylene glycol isobutyl ether (EIB) and water + diethylene glycol diethyl ether (DDE)). Adjusting one parameter, the theory yields satisfactory estimates of interfacial tensions as a function of temperature. Applying an equation of state, two variables (density and concentration) change in the interface. In this case the theory gives the density profiles of both components. The calculated density profiles demonstrate the ability of the concept to predict preferential adsorption phenomena in binary systems. The theoretical approach was applied to three systems (aniline + cyclohexane, acetone + carbon disulfide, and stearic acid + propane). If the systems show a closed miscibility gap, the interfacial tension passes a maximum. A comparison of theoretical and experimental interfacial tensions of the system aniline + cyclohexane indicates that the theoretical concept is able to describe the experiment if two parameters were fitted.

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The density gradient theory was combined with a modern equation of state based on statistical mechanics, the perturbed chain-statistical association fluid theory, or with the well-known Peng-Robinson equation of state, and is applied to compute the surface tension of various binary mixtures made up of nonpolar substances. Additionally, some new data for the system methane + hexane and methane + heptane were measured using the pendant drop method equipped with a high-pressure cell. Both equations of state can be used to predict the surface tension of mixtures very close to experimental data. The surface tension of pure ethane could also be predicted using the surface tension data of other n-alkanes. Additionally, the surface tensions for mixtures as a function of liquid composition, of temperature, and of pressure were calculated for systems, where no experimental data are available. If the mixture contains a high volatile component, strong relative enrichment effects in the interface could be observed.