Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal.
J Phys Chem B. 2012 Sep 6;116(35):10922-7. doi: 10.1021/jp306937f. Epub 2012 Aug 27.
This work reports the first data for the vapor pressures at several temperatures of the ionic liquids, [C(N/2)C(N/2)im][NTf(2)] (N = 4, 6, 8, 10, 12) measured using a Knudsen effusion apparatus combined with a quartz crystal microbalance. The morphology and the thermodynamic parameters of vaporization derived from the vapor pressures, are compared with those for the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series, [C(N-1)C(1)im][NTf(2)] (N = 3 - 9, 11, and 13). It was found that the volatility of [C(N/2)C(N/2)im][NTf(2)] series is significantly higher than the asymmetric cation ILs with the same total number of carbons in the alkyl side chains, [C(N-1)C(1)im][NTf(2)]. The observed higher volatility is related with the lower enthalpy of vaporization. The symmetric cation, [C(N/2)C(N/2)im][NTf(2)], presents lower entropies of vaporization compared with the asymmetric [C(N-1)C(1)im][NTf(2)], indicating an increase of the absolute liquid entropy in the symmetric cation ILs, being a reflection of a change of the ion dynamics in the IL liquid phase. Moreover both the enthalpy and entropy of vaporization of the [C(N/2)C(N/2)im][NTf(2)] ILs, present a clear odd-even effect with higher enthalpies/entropies of vaporization for the odd number of carbons in each alkyl chain ([C(3)C(3)im][NTf(2)] and [C(5)C(5)im][NTf(2)]).
这项工作报道了几种温度下离子液体[C(N/2)C(N/2)im][NTf(2)]（N = 4、6、8、10、12）蒸气压的首批数据，这些数据是使用带有石英晶体微天平的 Knudsen 蒸汽逸出装置测量得到的。从蒸气压得出的形态和汽化热力学参数与 1-烷基-3-甲基咪唑双（三氟甲烷磺酰基）亚胺系列[C(N-1)C(1)im][NTf(2)]（N = 3-9、11 和 13）的进行了比较。结果发现，[C(N/2)C(N/2)im][NTf(2)]系列的挥发性明显高于具有相同烷基侧链碳原子总数的不对称阳离子 ILs [C(N-1)C(1)im][NTf(2)]。观察到的较高挥发性与较低的汽化焓有关。与不对称的[C(N-1)C(1)im][NTf(2)]相比，对称阳离子[C(N/2)C(N/2)im][NTf(2)]的汽化熵较低，表明对称阳离子 IL 中绝对液体熵增加，这反映了 IL 液相中离子动力学的变化。此外，[C(N/2)C(N/2)im][NTf(2)] ILs 的汽化焓和熵都表现出明显的奇偶效应，即每个烷基链中碳原子数为奇数时具有较高的汽化焓/熵（[C(3)C(3)im][NTf(2)]和[C(5)C(5)im][NTf(2)]）。
