Pabst Florian, Gabriel Jan, Blochowicz Thomas
Institut für Festkörperphysik , Technische Universität Darmstadt , 64289 Darmstadt , Germany.
J Phys Chem Lett. 2019 May 2;10(9):2130-2134. doi: 10.1021/acs.jpclett.9b00686. Epub 2019 Apr 17.
Nanoscale structures in ionic liquids (ILs) are usually identified by X-ray or neutron scattering techniques and occur when the alkyl chains of the cations are long enough to show the tendency to segregate into apolar domains. In search of dynamic evidence for these nanostructures, different experimental techniques recently reported bimodal dynamic susceptibility spectra. In all cases, the faster process observed was ascribed to the structural α-relaxation and the slower one to the relaxation of long-lived aggregates. By contrast, we show by depolarized dynamic light scattering (DDLS) experiments on a systematic series of imidazolium-based ILs that the dynamics of the cation and anion are clearly separated for long alkyl chains. Therefore, the observation of a bimodal behavior is not related to any nanostructure but reflects the two-component nature of ILs. Thus, a consistent picture is obtained across different experimental methods, like dielectric and shear mechanical relaxation. Finally, the actual dynamic signature of nanostructures is identified for the first time as a weak feature in some of the DDLS spectra at even lower frequencies.
离子液体（ILs）中的纳米级结构通常通过X射线或中子散射技术来识别，当阳离子的烷基链足够长，表现出分离成非极性域的趋势时就会出现这种结构。为了寻找这些纳米结构的动态证据，最近不同的实验技术报道了双峰动态磁化率谱。在所有情况下，观察到的较快过程归因于结构α弛豫，较慢过程归因于长寿命聚集体的弛豫。相比之下，我们通过对一系列基于咪唑鎓的离子液体进行去偏振动态光散射（DDLS）实验表明，对于长烷基链，阳离子和阴离子的动力学明显分开。因此，双峰行为的观察与任何纳米结构无关，而是反映了离子液体的双组分性质。这样，通过不同的实验方法，如介电和剪切力学弛豫，得到了一致的结果。最后，纳米结构的实际动态特征首次被确定为在更低频率下一些DDLS光谱中的微弱特征。
