A semi-empirical equation for describing the surface tension of aqueous organic liquid mixtures

• There is no simple mixing rule to express the surface tension of ideal mixtures.
• Standard molar surface areas increase with increasing mixture surface tension.
• Model, error-free ideal surface tensions are also used to test empirical equations.
• The new equation is able to describe accurately aqueous organic surface tensions.

A thermodynamic analysis is made for the dependence on composition of the surface tension showing that the latter is a mechanical thermodynamic property which is linearly related to the reciprocal of each component standard molar surface area; and that there is no simple mixing formula linking the ideal surface tension to pure-component surface tensions.The ability of five current empirical equations to correlate the surface tension of the ideal and real binary systems water–ethanol or propan-2-ol or ethanenitrile or 1,4-dioxane at T = 298 K is examined. Polynomial equations are not able to describe the general, overall trend of surface tension variation across the entire composition range, where equations containing a hyperbolic term perform much better. This is the case of the proposed new equation in which two of its parameters can be estimated theoretically.

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