Vaporization enthalpy and the molar surface Gibbs free energy for ionic liquids [CnDmim][NTF2] (n = 2, 4)

•A new thermodynamic quantity-molar surface Gibbs free energy, g was put forward.•In terms of g, a new Eötvös empirical equation was derived.•The surface tensions for the two ILs and others could be predicted according to the new Eötvös equation.

Using thermogravimetrical analysis, the enthalpies of vaporization at an average temperature, ΔglHom(Tav), for ionic liquids (ILs) {1-alkyl-2,3-dimethyimidazolium-N,N-bis(trifluoromethyl sulfonyl)imide} [CnDmim][NTF2] (n = 2, 4) were firstly determined and transformed into ΔglHom(298 K) based on the ΔglCpom. Then, on the basis of Tong’s vaporization enthalpy model, a new thermodynamic quantity-molar surface Gibbs free energy, g was put forward and the values of g were calculated for [CnDmim][NTF2] (n = 2, 4). In terms of the molar surface Gibbs free energy, a new Eötvös empirical equation was derived so that the new Eötvös parameters have clear physical meanings: its slope is molar surface entropy, s, and the intercept is the molar surface enthalpy, h, the value of which is not change with temperature. Moreover, the values of molar surface Gibbs free energy can be estimated using the b and Tc easily, according to the new Eötvös equation and g = N1/3γV2/3, the surface tensions for [CnDmim][NTF2] (n = 2, 4) and other ILs could be predicted, and the predicted values were in good agreement with the experimental values.