A new model for describing the surface tension of binary liquid mixtures as a function of the bulk composition over the whole concentration range is presented. We first derive an equation relating surface and bulk volume fractions that generalizes the Langmuir isotherm so as to cover the entire range of concentrations. By combining this isotherm with a new mixing rule for nonideal solutions, we obtain an equation with two adjustable parameters, one measuring the lyophobicity of one component and the other accounting for the effect of molecular interactions. The model provides an excellent description of surface tension data for a wide variety of solutions with π0 = σA − σB values ranging from 2.2 to 51.0 mN/m.

Refractive indices (n), densities (F), and surface tensions (σ) for {formic acid, acetic acid, propionic acid, or butyric acid + water} mixtures at 298.15 K and normal atmospheric pressure have been determined as a function of mole fractions. From the experimental data, excess molar volumes (V E ) and deviations of refractive index from linear behavior (∆n φ ) have been calculated. The magnitude of these experimental quantities is discussed in terms of nature and type of intermolecular interactions in binary mixtures. Besides, the interpretation of the ratio of molar volume to molar refraction V/R as a measure of the degree of free volume appears to be a useful tool for qualitative considerations concerning volumetric and refractometric data. To analyze the behavior of surface tensions, the Extended Langmuir (EL) model was used.

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Adsorption at the liquid-vapor interphase of a liquid binary mixture is traditionally quantified by means of the Gibbs solute excess. Despite several theoretical reviews on the meaning of Gibbs excess defined by the Gibbs dividing surface, it is still misinterpreted as the excess concentration under Guggenheim's finite-depth surface layer approach. In this work, both concepts are clarified in a practical way, aided by a graphical representation without loss of generality. The understanding of both quantities led to the development of a thermodynamic procedure for the calculation of the actual number of solute and solvent molecules at a finite-depth surface layer (not a monolayer), what is called the absolute surface composition. From surface tension and density data, the absolute surface composition of the binary aqueous mixtures of methanol, ethanol, 1-propanol, and 1-butanol was calculated. Results show thermodynamic consistency and agree with experimental reports and with an empirical mixing rule. The increasing alcohol surface concentration throughout the entire concentration range casts doubt on the formation of an alcohol monolayer, as was suggested by other authors. Furthermore, the use of Guggenheim's monolayer model does not reproduce the experimental data, nor does it show thermodynamic consistency.

A method for calculating activity coefficients at infinite dilution from surface tension data is derived from the conditions for equilibrium between surface and bulk phases rather than from the conditions for liquid-vapor or liquid-liquid equilibrium, as is more usual. Specifically, the method combines the Volmer surface equation of state and the Gibbs adsorption equation to derive an expression for the surface chemical potential. The key operation of this treatment is the choice of the same reference state for both bulk and surface phases. The γ ∞ values calculated for systems for which suitable data are available in the literature agree well with values obtained by other methods.

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