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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