Study on thermodynamic properties and estimation of polarity of ionic liquids {[C<sub>n</sub>mmim][NTf<sub>2</sub>] (n = 2, 4)}
作者:Jie Wei、Tianyou Ma、Xiaoxue Ma、Wei Guan、Qingshan Liu、Jiazhen Yang
DOI:10.1039/c4ra04391j
日期:——
Two bis(trifluoromethyl sulfonyl)imide ionic liquids [Cnmmim][NTf2] (n = 2, 4) 1-alkyl-2,3-dimethyimidazolium-N,N-bis(trifluoromethyl sulfonyl)imide} were prepared and characterized by 1H NMR spectroscopy and differential scanning calorimetry (DSC). The values of their density, surface tension and refractive index were measured in the temperature range of (298.15 to 338.15 ± 0.01) K and the average contributions to the density, surface tension, and refractive index per methyl group in the alkyl chain and the addition of a methylene group in the imidazolium ring for the ILs were discussed. The dependence of volumetric properties, surface properties and molar refraction on temperature was discussed. Based on Kabo's method and Verevkin's experimental values, the molar enthalpies of vaporization, ΔHv, for [Cnmmim][NTf2] (n = 2, 4) were estimated. As a new idea, it was put forward that ΔHv can be assumed to consist of two parts: a part corresponds with the induced energy, ΔHvn, and another part corresponds with orientation energy from the permanent dipole moment of the ion pair in ILs, ΔHvμ. The values of ΔHvn were calculated in terms of the Lawson–Ingham equation so that the values of ΔHvμ could be estimated. Using the values of ΔHv, ΔHvn and ΔHvμ, cohesive energy density, δ2 (δ is Hildebrand solubility parameter), the contribution of induced energy, δn2, and the contribution of orientation energy, δμ2, were obtained. If a liquid only has δn then it is a non-polar liquid and if a liquid not only has δn, but also has δμ then it is a polar liquid. Since the ion pairs in ILs have a permanent dipole moment, the ionic liquid has a certain polarity. Therefore, using δμ as the polarity scaling of ILs is very convenient because the values of δμ are very easy to calculate from the enthalpy of vaporization and refractive index data.
制备并表征了两种双(三氟甲基磺酰)亚胺离子液体 [Cnmmim][NTf2](n = 2, 4)1-烷基-2,3-二甲基咪唑盐-N,N-双(三氟甲基磺酰)亚胺},采用了1H NMR光谱和差示扫描量热法(DSC)进行表征。在(298.15至338.15 ± 0.01)K的温度范围内测量了它们的密度、表面张力和折射率,并讨论了每个烷基链中的甲基对离子液体(ILs)密度、表面张力和折射率的平均贡献以及在咪唑环中加入一个亚甲基集团的影响。讨论了体积性质、表面性质和摩尔折射率对温度的依赖性。基于Kabo的方法和Verevkin的实验值,估算了 [Cnmmim][NTf2](n = 2, 4)的蒸发摩尔焓ΔHv。提出了一种新思路,即ΔHv可以认为由两部分组成:一部分对应于诱导能ΔHvn,另一部分对应于离子对中永久偶极矩的取向能ΔHvμ。利用Lawson–Ingham方程计算了ΔHvn的值,从而估算出ΔHvμ的值。通过ΔHv、ΔHvn和ΔHvμ的值,得到了内聚能密度δ2(δ是Hildebrand溶解度参数)、诱导能的贡献δn2和取向能的贡献δμ2。如果一种液体只有δn,那么它是非极性液体;如果一种液体不仅有δn,还有δμ,那么它是极性液体。由于离子液体中的离子对具有永久偶极矩,因此离子液体具有一定的极性。因此,使用δμ作为离子液体的极性标度非常方便,因为δμ的值可以很容易地从蒸发焓和折射率数据中计算得出。