Anomalous Solute Diffusivity in Ionic Liquids: Label-Free Visualization and Physical Origins
Anomalous Solute Diffusivity in Ionic Liquids: Label-Free Visualization and Physical Origins
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DOI：
10.1103/physrevx.9.011048
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发表时间：
2019-03-18
影响因子：
12.5
通讯作者：
Squires, Todd M.
中科院分区：
文献类型：
--
作者：
Bayles, Alexandra V.;Valentine, Connor S.;Squires, Todd M.
分子溶质在浓电解质中的动态扩散在许多应用中起着关键作用，但众所周知，测量和建模具有挑战性。这种挑战在离子液体（IL）的极端情况下尤其如此，离子液体完全由阳离子和阴离子组成。离子液体中的溶质扩散系数显示出强烈的浓度依赖性，拓宽了已经很大的离子液体设计空间，并使传统的，逐个样品的测量不切实际的筛选。为了更好地了解这类流体的运输机制，在这里，我们展示了一种方法来可视化浓度场的时空演变，使用微流体法布里-珀罗干涉法，使扩散率测量在一个单一的实验中的整个组成范围内。我们专注于水的吸收和扩散，作为一个模型溶质和无处不在的污染物，内alkylmethylimidazolium卤化物离子液体。值得注意的是，斯托克斯-爱因斯坦关系低估了水的扩散率10至50倍，表明水不经历这些离子液体作为连续液体。基于这些测量，再加上广角X射线散射和脉冲场梯度核磁共振测量，我们提出了一个新的机制框架，其中水分子之间的离子对之间的IL，它作为一个固定的矩阵超过相关的水扩散的时间尺度跳。在这种情况下，扩散是一个激活的过程，在氢键位点之间的跳跃超过一个能量屏障，随着水的分数线性下降。的活化能的函数形式是一致的NMR化学位移测量，这表明，氢键减弱成线性比例的水馏分。这个简单的模型包含的关键成分，需要准确地预测测得的扩散率的趋势（Arrhenius）的温度依赖性和指数组成的依赖性的范围内的阳离子，阴离子，水含量和温度。我们的研究结果表明，在离子液体中，溶质的结合位点之间的“跳跃”比离子重排更快的快速扩散的一般机制。
Dynamic diffusion of molecular solutes in concentrated electrolytes plays a critical role in many applications but is notoriously challenging to measure and model. This challenge is particularly true in the extreme case of ionic liquids (ILs), fluids composed entirely of cations and anions. Solute diffusivities in ILs show a strong concentration dependence, broadening the already vast IL design space and rendering conventional, sample-by-sample measurements impractical for screening. To gain better mechanistic insight into transport in this class of fluids, here we demonstrate a method to visualize the spatiotemporal evolution of concentration fields using microfluidic Fabry-Perot interferometry, enabling diffusivity measurements over an entire composition range within a single experiment. We focus on the absorption and diffusion of water, as both a model solute and a ubiquitous contaminant, within alkylmethylimidazolium-halide ILs. Notably, the Stokes-Einstein relation underpredicts water diffusivities ten- to 50-fold, indicating that water does not experience these ILs as continuum liquids. Based on these measurements, together with wide-angle x-ray scattering and pulsed-field gradient NMR measurements, we propose a new mechanistic framework in which water molecules hop between ion pairs within the IL, which acts as an immobile matrix over timescales relevant for water diffusion. In this case, diffusion is an activated process, with hops between hydrogen-bonding sites over an energetic barrier that decreases linearly with the water fraction. The functional form of the activation energy is consistent with NMR chemical shift measurements, which indicate that hydrogen bonding weakens in linear proportion to the water fraction. This simple model contains the key ingredients required to accurately predict the measured trends in diffusivity-an (Arrhenius) temperature dependence and an exponential composition dependence-for a range of cations, anions, water contents, and temperatures. Our results suggest a general mechanism for anomalously fast diffusion in ILs, where solutes "hop" between binding sites more quickly than the ions rearrange.