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

Abstract

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±0.21kJ·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.