Three pyridinium-based ionic liquids of [C(n)py][DCA] (n = 3,4,5) ( N-alkyl-pyridinium dicyanamide) were prepared and characterized by H-1 NMR (H-1 nuclear magnetic resonance) spectroscopy, C-13 NMR (C-13 nuclear magnetic resonance) spectroscopy. Their densities, surface tensions and refractive indices were measured at different temperatures. The molar surface Gibbs energy, g(s), critical temperature, T-c and E.tv.s empirical parameter related to polarity, k(E), were also calculated. In terms of the concept of molar surface Gibbs energy, g(s), a new E.tv.s equation was obtained. It is found that the slope of the new Eotvos 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 [C(n)py][DCA] (n = 3,4,5) are all most the same with the corresponding experimental values. (C) 2015 Elsevier Ltd. All rights reserved.