The molar surface Gibbs energy and prediction of surface tension of [Cnpy][DCA] (n = 3, 4, 5)

• The molar surface Gibbs energy, gs was put forword.
• A new Eötvös equation is obtained. The molar surface enthalpy, h, is a temperature-independent constant.
• By using gs and nD, the surface tensions of [Cnpy][DCA] (n = 3, 4, 5) were estimated.

Three pyridinium-based ionic liquids of [Cnpy][DCA] (n = 3, 4, 5) (N-alkyl-pyridinium dicyanamide) were prepared and characterized by 1H NMR (1H nuclear magnetic resonance) spectroscopy, 13C NMR (13C nuclear magnetic resonance) spectroscopy. Their densities, surface tensions and refractive indices were measured at different temperatures. The molar surface Gibbs energy, gs, critical temperature, Tc and Eӧtvӧs empirical parameter related to polarity, kE, were also calculated. In terms of the concept of molar surface Gibbs energy, gs, a new Eӧtvӧs equation was obtained. It is found that the slope of the new Eӧtvӧs equation is the molar surface entropy of the ILs and the intercept is the molar surface enthalpy which is a temperature-independent constant. By using the refractive index and the molar surface Gibbs energy, an equation to predict surface tension of the ILs was derived and the predicted values of the surface tension of [Cnpy][DCA] (n = 3, 4, 5) are all most the same with the corresponding experimental values.