Institut für Festkörperphysik, Technische Universität Darmstadt, Hochschulstr. 6, 64289 Darmstadt, Germany.
J Chem Phys. 2018 Sep 14;149(10):104501. doi: 10.1063/1.5047825.
We combine H, H, and Li NMR methods to investigate the dynamics of water molecules and lithium ions in LiCl aqueous solutions over wide ranges of time and length scales down to their glass transitions. Structural relaxation times and self-diffusion coefficients reveal that water and lithium dynamics are faster for lower salt content at ambient temperatures, while the differences vanish upon cooling when fractional freezing leads to similar salt concentrations in the remaining liquid phases. Relaxation times and diffusion coefficients of water molecules agree with those of lithium ions in the weakly supercooled regime, indicating that the dynamics are strongly coupled. Furthermore, non-Arrhenius temperature dependence is found and the Stokes-Einstein relation is obeyed in this temperature range. However, we observe various decoupling phenomena for the motion of the constituents and for dynamics on different length scales in the deeply supercooled regime. Most notably, the rotational motion of the water molecules does not follow the glassy slowdown of the studied salt solutions below ∼145 K, but it rather resembles that in nanoscopic confinement, molecular solutions, and high-density amorphous ice at low temperatures. This common low-temperature water dynamics is characterized by large-angle reorientation and Arrhenius temperature dependence.
我们结合 H、H 和 Li NMR 方法，在很宽的时间和长度范围内研究了水和锂离子在氯化锂水溶液中的动力学，直至其玻璃化转变。结构弛豫时间和自扩散系数表明，在环境温度下，较低盐含量的水和锂离子动力学更快，而当分数冷冻导致剩余液相中具有相似的盐浓度时，冷却时差异消失。在弱过冷区，水分子的弛豫时间和扩散系数与锂离子的一致，表明动力学强烈耦合。此外，在该温度范围内发现了非 Arrhenius 温度依赖性，并且遵循 Stokes-Einstein 关系。然而，我们在过冷区观察到各组分的运动和不同长度尺度上的动力学存在各种解耦现象。最值得注意的是，在约 145 K 以下，水分子的旋转运动没有遵循所研究盐溶液的玻璃化减速，而是类似于在纳米受限、分子溶液和高密度非晶冰中的低温旋转运动。这种常见的低温水动力学的特征是大角度重取向和 Arrhenius 温度依赖性。
