Ionic liquids based on diethylmethyl(2-methoxyethyl)ammonium cations and bis(perfluoroalkanesulfonyl)amide anions: influence of anion structure on liquid properties
作者:Yukihiro Yoshida、Gunzi Saito
DOI:10.1039/c1cp21783f
日期:——
A series of diethylmethyl(2-methoxyethyl)ammonium (DEME)-based ionic liquids were prepared using bis(perfluoroalkanesulfonyl)amide (CnF2n+1SO2)2N anions with different perfluoroalkyl chain lengths (n = 0, 1, 2, 3, and 4), and the influence of the structural variation on their thermal, ion-diffusive (ionic conductivity and viscosity), ion-concentration (molar concentration and ion association), and solvatochromic (polarity and hydrogen-bond acceptor ability) properties was investigated. The elongation of the perfluoroalkyl chain causes the pronounced suppression of ionic conductivity, fluidity, and polarity. According to the crystallographic study of the corresponding (CnF2n+1SO2)2N salts formed with high-symmetrical tetramethylammonium cations, the decreased ion diffusivity must be a consequence of the increased contribution of the interionic van der Waals interactions of F⋯F type and hydrogen-bonding interactions of C–H⋯F type in addition to C–H⋯O type. The Kamlet–Taft π*-scale (polarity) is under the control of the ion concentration, associated with the perfluoroalkyl chain length in the anions. The larger Kamlet–Taft β-scale (hydrogen-bond acceptor ability) of DEME-based ionic liquids with a longer perfluoroalkyl chain appears to be responsible for the larger degree of ion association of oppositely charged ions, which was manifested in the Walden rule deviation.
使用具有不同全氟烷基链长度(n = 0、1、2、3 和 4)的双(全氟烷磺酰)酰胺(CnF2n+1SO2)2N 阴离子制备了一系列基于二乙基甲基(2-甲氧基乙基)铵(DEME)的离子液体、并研究了结构变化对它们的热、离子扩散(离子传导性和粘度)、离子浓度(摩尔浓度和离子关联)和溶解变色(极性和氢键接受能力)特性的影响。全氟烷基链的拉长导致离子导电性、流动性和极性明显减弱。根据对高对称性四甲基铵阳离子形成的相应 (CnF2n+1SO2)2N 盐的结晶学研究,离子扩散性降低的原因一定是 F⋯F 型离子间范德华相互作用和 C-H⋯O 型以外的 C-H⋯F 型氢键相互作用的贡献增加了。Kamlet-Taft π*尺度(极性)受离子浓度控制,与阴离子中的全氟烷基链长度有关。全氟烷基链较长的基于 DEME 的离子液体的 Kamlet-Taft β 级(氢键接受能力)较大,这似乎是带相反电荷的离子的离子结合程度较大的原因,表现为瓦尔登规则偏差。