Study On Enthalpy Of Vaporization, Polarity And Molar Surface Gibbs Free Energy For Ioinc Liquids

Three different kinds of ionic liquids,1-alkyl-3-methyllimidazolium acetate [Cnmim][OAc](n= 2,4,5,6) with simple synthetic method and good biological compatibility,1-alkyl-3-methyllimidazolium lactic [Cnmim][Lact](n= 2,3,4,5) with chiral structure and l-alkyl-2,3-dimethyimidazolium-N,N-bis(trifluoromethyl sulfonyl)imide [CnDmim][NTF2](n= 2,4,6) with good thermal stability, are carried on comprehensively studied in this work. The physical-chemical properties such as the vaporization enthalpy, the difference in heat capacity of gaseous state and liquid, entropy vaporization, Gibbs free energy, polarity and molar surface Gibbs free energy.Firstly, eleven ionic liquids, [Cnmim][OAc](n= 2,4,5,6), [Cnmim][Lact](n = 2,3,4,5) and [CnDmim][NTF2](n = 2,4,6) were prepared and confirmed by 1H-NMR spectroscopy,13C-NMR spectroscopy, and differential scanning calorimetry (DSC).Secondly, using thermogravimetrical analysis, the enthalpies of vaporization at an average temperature, △g1Hom(Tav), for ionic liquids [Cnmim][OAc](n= 2,4,5,6), [Cnmim][Lact](n= 2,3,4,5) and [CnDmim][NTF2](n= 2,4,6) were determined respectively. The difference of heat capacities between the vapor phase and the liquid phase, Agg1Cp,m, for the ionic liquids were calculated based on the statistical thermodynamics, and then in terms of Ag1Cp,m, the values of △g1Hom(Tav) can be transformed into △g1Hom(298). The vaporization enthalpies for [Cnmim][OAc](n= 2,4,6) were also further tested by molecular dynamic (MD) simulations. Comparison between the two results, they are in a good agreement. Simultaneously, vaporization enthalpy, vapour pressure, entropy vaporization and Gibbs free energy at different temperatures were also calculated and the relationships between these properties and temperatures were discussed. In terms of Hildebrand theory, the new scale of polarity for ILs was put forward base on the values of enthalpies of vaporization, and the order of the polarity of the ILs is also predicted. It turns out that the order using Reichardt’s are in good agreement with one using the new scale of polarity.Finally, a new concept-molar surface Gibbs free energy, gs, was put forward in terms of the model of enthalpy of vaporization, and the values of gs were calculated for [Cnmim][OAc](n= 2,4,5,6), [Cnmim][Lact](n= 2,3,4,5) and [CnDmim][NTF2](n= 2,4,6). The vaporization enthalpies were predicted according to the relationship between gs and vaporization enthalpy; In terms of gs, the traditional Eotvos equation was improved, which the 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 does not change with temperatures, that is, the molar surface heat capacity cp= 0 implies that the process from the inside to the surface of the liquid is an isocoulombic one. These facts show that new Eotvos equation is not only an empirical equation, but also a strict thermodynamic equation. In addition, combined gs with Lorentz-Lorenz equation, an expression for predicting surface tension of the ILs was obtained, and the predicted values of surface tension are in good agreement with the experimental ones.