A Molecular Level Understanding of Template Effects in Ionic Liquids.

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.