Mulliken Center for Theoretical Chemistry, University of Bonn , Beringstr. 4+6, D-53115 Bonn, Germany.
Max Planck Institute for Chemical Energy Conversion , Stiftstr. 34-36, D-45413 Mülheim an der Ruhr, Germany.
Acc Chem Res. 2017 Dec 19;50(12):2949-2957. doi: 10.1021/acs.accounts.7b00436. Epub 2017 Nov 13.
The structure-directing or template effect has been invoked several times for ionic liquids to explain the different outcome in material synthesis, namely, different scaffolds or geometrical arrangements with varying ionic liquids. It is obvious to assume that such an effect can originate from the most likely complex microstructure, being present within the ionic liquid itself. In that regard, ionic liquids have already been shown to undergo a nanosegregation into polar and nonpolar phases, which is commonly known and denoted as microheterogeneity. In order to provide detailed insight on the molecular level and to understand the effects rising from this structuring, we performed molecular dynamics simulations on selected very simple model systems composed of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, considering ethyl, butyl, hexyl, and octyl side chains attached to the cations, mixed with either n-dodecanol or n-butanol. By analyzing snapshots of the simulation boxes and calculating spatial distribution functions, we can visualize that with increasing side chains, the systems show considerable nanosegregation into polar and nonpolar domains. Combined angular and distance distribution functions show that in case of the nanosegregating systems the side chains of the cations are preferentially arranged in a parallel fashion, which indicates a micelle-like structure for the ionic liquids. The alcohol molecules participate in and are, therefore, influenced by this microheterogeneity. It can be shown that in the case of the short IL alkyl side chains, the self-aggregation of the nonpolar units of the alcohols is much stronger, while for the long chain cations, the nonpolar entities of the alcohols are most often connected to the nonpolar units of the ionic liquids. Using our domain analysis tool, we can quantify these observations by tracking the number, size, and shape of the polar and nonpolar entities present in the different investigated systems. The aforementioned combined angular-distance distribution functions reveal a structure-directing effect of the ionic liquids on the alcohol molecules within our simple model systems. The ionic liquids act as template and order the alcohol molecules according to their own structure, resulting in a parallel alignment of the alkyl side chains of the alcohols and ionic liquid cations, with both polar groups being at the same side. These observations show that the microheterogeneous structure of ionic liquids can indeed be applied to order substrates with respect to each other or, for example, to catalysts in a predetermined fashion, opening new possibilities for explaining or enhancing selectivities of chemical reactions in ionic liquids.
结构导向或模板效应在离子液体中多次被调用，以解释在材料合成中不同的结果，即具有不同离子液体的不同支架或几何排列。显然可以假设，这种效应可能源于最可能的复杂微观结构，存在于离子液体本身中。在这方面，已经表明离子液体经历纳米分离为极性和非极性相，这是常见的，称为微不均匀性。为了在分子水平上提供详细的见解，并理解由此产生的结构的影响，我们对由 1-烷基-3-甲基咪唑鎓双（三氟甲烷磺酰基）亚胺组成的选定非常简单的模型系统进行了分子动力学模拟，考虑了与阳离子连接的乙基、丁基、己基和辛基侧链，并与正十二醇或正丁醇混合。通过分析模拟盒的快照并计算空间分布函数，我们可以直观地看到，随着侧链的增加，系统显示出明显的极性和非极性域的纳米分离。组合角和距离分布函数表明，在纳米分离系统中，阳离子的侧链优先以平行方式排列，这表明离子液体具有胶束样结构。醇分子参与其中，因此受到这种微不均匀性的影响。可以表明，在短 IL 烷基侧链的情况下，醇的非极性单元的自聚集要强得多，而对于长链阳离子，醇的非极性实体最常与离子液体的非极性单元连接。使用我们的域分析工具，我们可以通过跟踪不同研究系统中存在的极性和非极性实体的数量、大小和形状来量化这些观察结果。上述组合角距离分布函数揭示了离子液体对我们简单模型系统中醇分子的结构导向效应。离子液体充当模板，根据自身结构对醇分子进行排序，导致醇的烷基侧链和离子液体阳离子平行排列，极性基团位于同一侧。这些观察结果表明，离子液体的微不均匀结构确实可以用于彼此有序排列底物，或者例如以预定方式排列催化剂，为解释或增强离子液体中化学反应的选择性开辟了新的可能性。
