The molar surface Gibbs free energy and its application 2: Ionic liquids 1-alkyl-3-methylimidazolium threonine salts [C_nmim][Thr] (n = 2, 4) at T = (288.15 to 328.15) K

Graphical abstract: A new Eötvös equation, g = B₁(T_c − T), was obtained, where g is the molar surface Gibbs energy; B₁ = s is the molar surface entropy; the product of B₁ with critical temperature, T_c, is the molar surface enthalpy, h, which is a temperature-independent constant. - Highlights: • The new concept of molar surface Gibbs free energy, g, was proposed. • Using g, the traditional Eötvös equation was improved to be a new one. • An equation to predict surface tension of ILs is derived with refractive index and g. - Abstract: Two new amino acid ionic liquids (AAILs) [C_nmim][Thr] (n = 2, 4) (1-alkyl-3-methylimidazolium threonine salt) were prepared and characterized. The values of density, surface tension and refractive index of the ILs were measured at T = (288.15 to 328.15) K and the values of the molar surface Gibbs free energy, g, were calculated. Using the molar surface Gibbs free energy, the traditional Eötvös equation was improved to be a new Eötvös equation. It is found that the slope of the new Eötvös equation is the molar surface entropy of the ILs and the intercept is the molar surface enthalpy which is a temperature-independent constant. With the help of the refractive index and the molar surface Gibbs free energy, an equation to predict surface tension of the ILs was derived. Using this equation, predicted values of surface tension with the corresponding experimental ones are highly correlated and extremely similar. According to a new scale of polarity for ILs, the polarity of ionic liquids [C_nmim][Thr] (n = 2, 4) was estimated.