Association and molecular chain length effects on interfacial behavior

The scope of this work is to analyze the influence of the molecular chain length and association effects on the properties of vapor-liquid interfaces. Calculations are based on the gradient theory applied to the Statistical Associated Fluid Theory (SAFT) equation of state (EOS), yielding thus an approach that predicts both phase equilibrium and interfacial properties. In addition, the theoretical structure of the SAFT-EOS includes the specific effects under consideration. The approach proposed here is coherent with the density functional theory, although it is more direct to apply, and predictions are in good agreement with experimental data. Results show that the interface thickness decreases, while the interfacial tension increases, as the molecular chain length and/or the association degree increases at isothermal conditions. Such a trend may be explained in terms of the distortion of the cohesion energy. Detailed examples are discussed for subcritical binary mixtures and predictions are confronted with experimental data.