Wang Huiyong, Wang Jianji, Zhang Sheli, Pei Yuanchao, Zhuo Kelei
School of Chemical and Environmental Sciences, Henan Key Laboratory of Environmental Pollution Control, Henan Normal University, Xinxiang, Henan 453007, P. R. China.
Chemphyschem. 2009 Oct 5;10(14):2516-23. doi: 10.1002/cphc.200900438.
Considering the ionic nature of ionic liquids (ILs), ionic association is expected to be essential in solutions of ILs and to have an important influence on their applications. Although numerous studies have been reported for the ionic association behavior of ILs in solution, quantitative results are quite scarce. Herein, the conductivities of the ILs [Cnmim]Br (n=4, 6, 8, 10, 12), [C4mim][BF4], and [C4mim][PF6] in various molecular solvents (water, methanol, 1-propanol, 1-pentanol, acetonitrile, and acetone) are determined at 298.15 K as a function of IL concentration. The conductance data are analyzed by the Lee-Wheaton conductivity equation in terms of the ionic association constant (KA) and the limiting molar conductance (Lambda(m)(0)). Combined with the values for the Br- anion reported in the literature, the limiting molar conductivities and the transference numbers of the cations and [BF4]- and [PF6]- anions are calculated in the molecular solvents. It is shown that the alkyl chain length of the cations and type of anion affect the ionic association constants and limiting molar conductivities of the ILs. For a given anion (Br-), the Lambda(m)(0) values decrease with increasing alkyl chain length of the cations in all the molecular solvents, whereas the KA values of the ILs decrease in organic solvents but increase in water as the alkyl chain length of the cations increases. For the [C4mim]+ cation, the limiting molar conductivities of the ILs decrease in the order Br- > [BF4]- > [PF6]-, and their ionic association constants follow the order [BF4]- > [PF6]- > Br- in water, acetone, and acetonitrile. Furthermore, and similar to the classical electrolytes, a linear relationship is observed between ln KA of the ILs and the reciprocal of the dielectric constants of the molecular solvents. The ILs are solvated to a different extent by the molecular solvents, and ionic association is affected significantly by ionic solvation. This information is expected to be useful for the modulation of the IL conductance by the alkyl chain length of the cations, type of anion, and physical properties of the molecular solvents.
考虑到离子液体（ILs）的离子性质，离子缔合在离子液体溶液中应是至关重要的，并且对其应用有重要影响。尽管已有大量关于离子液体在溶液中的离子缔合行为的研究报道，但定量结果却相当稀少。在此，测定了离子液体[Cnmim]Br（n = 4、6、8、10、12）、[C4mim][BF4]和[C4mim][PF6]在各种分子溶剂（水、甲醇、1 - 丙醇、1 - 戊醇、乙腈和丙酮）中于298.15 K时作为离子液体浓度函数的电导率。根据离子缔合常数（KA）和极限摩尔电导率（Λm(0)），通过Lee - Wheaton电导率方程对电导数据进行分析。结合文献中报道的Br - 阴离子的值，计算了分子溶剂中阳离子以及[BF4]-和[PF6]-阴离子的极限摩尔电导率和迁移数。结果表明，阳离子的烷基链长度和阴离子类型会影响离子液体的离子缔合常数和极限摩尔电导率。对于给定的阴离子（Br - ），在所有分子溶剂中，Λm(0)值随阳离子烷基链长度的增加而降低，而离子液体的KA值在有机溶剂中降低，但在水中随阳离子烷基链长度的增加而增加。对于[C4mim]+阳离子，离子液体的极限摩尔电导率按Br - > [BF4]- > [PF6]-的顺序降低，其离子缔合常数在水、丙酮和乙腈中按[BF4]- > [PF6]- > Br - 的顺序排列。此外，与经典电解质类似，观察到离子液体的ln KA与分子溶剂介电常数的倒数之间存在线性关系。离子液体被分子溶剂不同程度地溶剂化，离子缔合受到离子溶剂化的显著影响。这些信息有望用于通过阳离子的烷基链长度、阴离子类型和分子溶剂的物理性质来调节离子液体的电导。
