A new theoretical model - The ionic molar surface Gibbs free energy and its application

- Five ether-functionalized ionic liquids (ILs) are prepared and characterized.
- Their density and surface tension are measured by SAM at T = 298.15-338.15 K.
- A new theoretical model, the ionic molar surface Gibbs free energy, is proposed.
- For 50 ionic liquids, the predictions agree well with experiments.

Three novel ether-functionalized ionic liquids (ILs) 1-ethyl-3-(2-methoxyethyl)-imidazolium acetate, [C22O1IM][OAc], 1-ethyl-3-(2-methoxyethyl)-imidazolium thiocyanate, [C22O1IM][SCN], 1-ethyl-3-(2-methoxyethyl)-imidazolium trifluoromethanesulfonate, [C22O1IM][TfO], and two ILs 1-ethyl-3-(2-methoxyethyl)-imidazolium dicyanamide, [C22O1IM][DCA], 1-ethyl-3-(2-methoxyethyl)-imidazolium bis(trifluoromethylsulfonyl)imide, [C22O1IM][NTF2] were prepared and characterized. Their density and surface tension were measured at T = (298.15-338.15) K. As a new theoretical model, the ionic molar surface Gibbs free energy, gi, was put forward. The ionic molar surface Gibbs free energy, g−, for [BF4]−, which was as reference ion, was obtained in terms of the extrathermodynamic assumptions so that the ionic molar surface Gibbs free energy, g+, of the corresponding cations, [Cnmim]+ (n = 2-6), were determined. In terms of the g+ values of [Cnmim]+ (n = 2-6), the g− values of 12 kinds of anions were determined and then the average g+ value of [C22O1IM]+ (9.52 ± 0.21 kJ·mol−1) was gotten. By using the values of these gi, the predicted values of molar surface Gibbs free energy, gest, and surface tension, γest, for [C22O1IM][X] (X = OAC, SCN, DCA, TfO, NTF2) and other 45 ionic liquids were estimated. In comparison, the predicted values correlated quite well with their matching experimental values.

The estimated molar surface Gibbs free energy and the corresponding experimental values are highly correlated (correlation coefficient squared, r = 0.990, standard deviation, sd = 0.16) and extremely similar (gradient = 1.02; intercept = −0.25). Plot of the estimated molar surface Gibbs free energy, gest, for ILs vs. their experimental values, gexp, y = −0.25 + 1.02 x, sd = 0.16, r = 0.990.57