Study on thermophysical properties for aqueous solutions of ionic liquid N-ethyl-pyridine dicyanamide [C2py][DCA]

- The values of ρ and γ for the IL [C2py][DCA] were obtained by SAM.
- The values of ρ and γ for aqueous solutions of [C2py][DCA] were determined.
- The molar surface Gibbs free energy model of the IL solution was proposed.
- A new Eötvös equation was obtained.

The IL (ionic liquid) [C2py][DCA] (N-ethyl-pyridine dicyanamide) was prepared and confirmed by 1H NMR (1H nuclear magnetic resonance) spectroscopy, 13C NMR (13C nuclear magnetic resonance) spectroscopy. The values of density and surface tension of the IL [C2py][DCA] and its aqueous solutions with various molalities were determined at T = (288.15 to 318.15) K. The values of the molar volume for aqueous solutions of [C2py][DCA], V, were calculated and increase with the increase of temperature and molarity. On the basis of thermodynamics, a semi-empirical model-molar surface Gibbs free energy model of the ionic liquid solution was put forward. It can be used to predict the surface tension and the predicted values of the surface tension for aqueous [C2py][DCA] are all most the same with the corresponding experimental ones. In terms of the concept of the molar surface Gibbs free energy, a new Eötvös equation is obtained. Each parameter of the new equation has a clear physical meaning, energy dimension and is more convenient to use.

Fitting values of γ(est.) predicted by the molar surface Gibbs free energy model vs corresponding experimental one, γ(exp.), a good straight line was obtained and it can be seen that predicted surface tensions and the corresponding experimental values are highly correlated (correlation coefficient squared, r = 0.999, standard deviation, s = 0.180) and extremely similar (gradient = 0.999; intercept = 0.0854).141