Solute Diffusion in Ionic Liquids, NMR Measurements and Comparisons to Conventional Solvents
Solute Diffusion in Ionic Liquids, NMR Measurements and Comparisons to Conventional Solvents
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DOI：
10.1021/jp405393d
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发表时间：
2013-10-03
影响因子：
3.3
通讯作者：
Maroncelli, Mark
中科院分区：
文献类型：
--
作者：
Kaintz, Anne;Baker, Gary;Maroncelli, Mark
1-烷基-1-甲基吡咯烷鎓双(三氟甲磺酰基)亚胺系列 ([Pr-n1][Tf2N], n = 3, 4, 6, 8, 和 10)、三己基十四烷基鏻双(三氟甲磺酰基)亚胺系列中各种稀溶质的扩散系数使用脉冲场梯度 H-1 NMR 测量 [P-14、P-666][Tf2N] 和各种咪唑鎓离子液体。这些数据与现有文献数据相结合，用于尝试揭示对于确定示踪剂扩散速率最重要的溶质和溶剂特征。讨论的框架是使用 zeta(obs)/zeta(SE) 比率对平移摩擦进行简单的流体动力学预测，其中 zeta(obs) 是观察到的摩擦力，通过测量的扩散系数 D 通过 zeta(obs) = k(B)T/D 确定，而 delta(SE) = 6 pi eta R 是在具有粘度的溶剂中半径为 R 的球体（由溶质范德华体积确定）上的斯托克斯摩擦力预计。对于中性溶质，流体动力学预测是否准确的主要决定因素是溶质与溶剂分子的相对大小。 zeta(obs)/zeta(SE) 与溶质与溶剂范德华体积之比之间存在单一相关性，尽管分散程度相当大，zeta(obs)/zeta(SE) = {1 + a(V-U/V-V)(-P)}，常数 a = 1.93，p = 1.88。对于小溶质，观察到的摩擦比流体动力学模型的预测小 100 倍以上。溶质的偶极矩对摩擦力的影响很小，而溶质的电荷则有显着的影响。对于尺寸与溶剂离子相当或小于溶剂离子的一价溶质，观察到的摩擦力与流体动力学预测的摩擦力相当甚至更大。这些总体趋势与传统溶剂中示踪剂扩散的观察结果非常相似。
Diffusion coefficients of a variety of dilute solutes in the series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imides ([Pr-n1][Tf2N], n = 3, 4, 6, 8, and 10), trihexyltetracedecylphosphonium bis(trifluoromethanesulfonyl)imide [P-14,P-666][Tf2N], and assorted imidazolium ionic liquids are measured using pulsed field gradient H-1 NMR. These data, combined with available literature data, are used to try to uncover the solute and solvent characteristics most important in determining tracer diffusion rates. Discussion is framed in terms of departures from simple hydrodynamic predictions for translational friction using the ratio zeta(obs)/zeta(SE), where zeta(obs) is the observed friction, determined from the measured diffusion coefficient D via zeta(obs) = k(B)T/D, and delta(SE) = 6 pi eta R is the Stokes friction on a sphere of radius R (determined from the solute van der Waals volume) in a solvent with viscosity eta. In the case of neutral solutes, the primary determinant of whether hydrodynamic predictions are accurate is the relative size of solute versus solvent molecules. A single correlation, albeit with considerable scatter, is found between zeta(obs)/zeta(SE) and the ratio of solute-to-solvent van der Waals volumes, zeta(obs)/zeta(SE) = {1 + a(V-U/V-V)(-P)}, with constants a = 1.93 and p = 1.88. In the case of small solutes, the observed friction is over 100 fold smaller than predictions of hydrodynamic models. The dipole moment of the solute has little effect on the friction, whereas solute charge has a marked effect. For monovalent solutes of size comparable to or smaller than the solvent ions, the observed friction is comparable to or even greater than what is predicted by hydrodynamics. These general trends are shown to be quite similar to what is observed for tracer diffusion in conventional solvents.