Study Onthermodynamic Properties Of N-alkyl Pyridiniumium Ionic Liquids

Ionic liquids are not only environmentally friendly green solvents,highly efficient catalyst and extractant in the chemical field,but also have the widespread application prospect in the medical field of drug solubilizationwith its unique excellent performance,such as the low saturated vapor pressure,the wide liquid temperature range and the functional designability.In addition,the ionic liquid as an electrolyte in the battery system has also been a breakthrough.It can be seen that,as apotentially emerging industry,ionic liquids have demonstrated outstanding potential value in people's growing demand and ever-changing production with the aim of meeting new opportunities and greater challenges in prominent problems for development of renewable and new energy.Two series of pyridinium-based ionic liquids,[CnPy][OAc](N-alkylpyridinium acetate)and [CnPy][TFA](N-alkylpyridinium trifluoroacetate)(n = 2-6)were designed and synthesized by the neutralization method and these ten products were characterized by 1H NMR and DSC.Since the anions CH3COO-and CF3COO-are easy to form hydrogen bonds with polar solvent water and impurity water can interfere with the measurement of thermodynamic properties.The standard addition method(SAM)was applied to the measurements of the physical properties of the two series of ionic liquids.Firstly,the thermodynamic properties of these ionic liquids with different water contents were measured,and the properties of pure ionic liquids were obtained by linear fitting and the application of SAM in the determination effectively overcomes the interference of impurity water.The density,surface tension and refractive index of the ionic liquids [CnPy][OAc] and [CnPy][TFA](n = 2-6)with different water contents were measured in the temperature range of(293.15 to 343.15 ± 0.1)K with an interval of 5K.On the basis of these experimental datas,the other volume properties and surface properties of the two serires of ionic liquids were estimated by the semi empirical method.Firstly,the molecular volume,Vm,of the ionic liquids was calculated and the changes of molecular volume with temperature and the number of carbon atoms in the alkyl chain were investigated.According to the Glasser theory,the standard entropy,S(298),and the lattice energy,,are calculated,and the linear variation of the standard entropy aginst the number of carbon atoms in the alkyl chain is analyzed and the contribution of each methylene to the standard entropy is obtained.By comparing the lattice energy of the ionic liquids and inorganic molten salt,the reason why the ionic liquid is called molten salt at room temperature is clear.In addition,the surface entropy,Sa,and surface energy,Ea,of the ionic liquids were calculated,and it was found that the surface energy was very small compared with the inorganic molten salt,thatexplains why ionic liquids can remain liquid state at room temperature.The enthalpy of vaporization of the ionic liquids[CnPy][OAc] and [CnPy][TFA] was calculated by the empirical equations of Kabo and Rebelo and the possibility of differences in results was analyzed.Using the concept of molar surface Gibbs energy,the traditional E?tv?s equation was improved into a new E?tv?s equation in which the intercept and the slope represent molar surface enthalpy and molar surface entropy for the ionic liquids,separately.In terms of the refractive index and the molar surface Gibbs free energy,an equation to predict surface tension of the ionic liquids was derived and the predicted values are correlated and are in good agreement withthe corresponding experimental one.According to interstice model,the thermal expansion coefficient of the two series of ionic liquids,?,were calculated and the magnitude order is well consistent with the corresponding experimental value and a new concept is proposed,that is the interstice molar surface Gibbs free energy,gs,it's a constant,which has nothing to do with the types of ionic liquids.In addition,based on Hildebrand's theory,a new standard,the solubility parameters,??,was proposed to describe the polarity of the ionic liquidsandthe results of hydrophilicity and hydrophobicity are accordant with our daily experience.