Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States.
J Phys Chem B. 2011 Apr 28;115(16):4576-82. doi: 10.1021/jp1110899. Epub 2011 Apr 4.
In order to understand the unique solvation and conduction properties of ionic liquids (ILs), we explore their interionic associations modulated by hydration level and ionic medium. Pulsed-field-gradient NMR allows sensitive measurement of separate cation and anion diffusion coefficients, which combine to reflect ionic aggregation. With increasing hydration of ILs, the anomalous ratio of cation to anion diffusion coefficients reverses, then plateaus to values consistent with expected hydrodynamic radii ratios (r(cation)/r(anion) = 1.4 for [C(2)mim][BF(4)]). When ILs diffuse inside an ionic polymer, ion associations are modulated by ionic interactions between mobile cations and anions, and drag from fixed -SO(3)(-) lining the polymer's hydrophilic channels. Surprisingly, cations diffuse substantially faster (≤3×) at low hydration inside membranes, revealing prevalent anionic aggregates. At high hydration, isolated anions diffuse faster (≤4×) than cations. Probing ionic interactions provides pivotal insight into these subtle fluids, with quantitative implications for electrolyte applications such as batteries and "artificial muscle" mechanical actuators.
为了理解离子液体（ILs）独特的溶剂化和传导性质，我们探索了其在水合水平和离子介质调节下的离子间相互作用。脉冲梯度 NMR 允许对单独的阳离子和阴离子扩散系数进行灵敏测量，它们的组合反映了离子聚集。随着 ILs 水合程度的增加，阳离子与阴离子扩散系数的异常比值发生反转，然后趋于与预期的流体力学半径比值（对于 [C(2)mim][BF(4)]，r(cation)/r(anion) = 1.4）一致。当 ILs 在离子聚合物内扩散时，离子相互作用会调节移动阳离子和阴离子之间的离子缔合，以及聚合物亲水区段的固定 -SO(3)(-) 带来的拖拽。令人惊讶的是，在低水合条件下，阳离子在膜内的扩散速度明显更快（≤3×），表明存在普遍的阴离子聚集。在高水合条件下，阴离子的扩散速度（≤4×）比阳离子快。探究离子相互作用为这些微妙的流体提供了关键的见解，对电池和“人造肌肉”机械致动器等电解质应用具有定量意义。
