A Molecular Level Understanding of Template Effects in Ionic Liquids

Abstract

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.

Strong Microheterogeneity in Novel Deep Eutectic Solvents

“…Similarly, the long alkyl side chain is counted as nonpolar group. With this distinction we always obtain one polar completely connected domain (domain count=1), see in Table the 4th column P, which is in agreement with the behavior of IL systems . All nonpolar groups are heavily dispersed in the polar parts for side chain length of less or equal than four carbon units, see entry for DES containing the [C 4 Ch] + and the [ChC 4 OH] + cation 3rd column in Table .…”

“…With this distinction we always obtain one polar completely connected domain (domain count = 1), see in Table 1 the 4th column P, which is in agreement with the behavior of IL systems. [33,34,45] All nonpolar groups are heavily dispersed in the polar parts for side chain length of less or equal than four carbon units, see entry for DES containing the [C 4 Ch] + and the [ChC 4 OH] + cation 3rd column in Table 1. From the side chain length of eight carbon units, the nonpolar parts of the DES stick together as well and real MH can be observed in that the polar and the nonpolar parts each form one fully connected microphase which shows that in order 3 Ch] + .…”

“…[28] So far only a few studies about polar-nonpolar microheterogeneity (MH) -which is the separation of polar and nonpolar molecular parts - [29] has occurred in literature. However, microheterogeneity has been extensively investigated in ILs [12,[30][31][32][33][34] and recently in the context of the template or structure-directing effects in ILs. [34] As template assisted syntheses have been carried out in DESs as well [35,36] and this can be often connected to microheterogeneity, suggestions for DESs which exhibit strong MH are desirable.MH studies in DES started when hydrophobic DES were presented and successfully applied in extraction processes.…”

“…However, microheterogeneity has been extensively investigated in ILs [12,[30][31][32][33][34] and recently in the context of the template or structure-directing effects in ILs. [34] As template assisted syntheses have been carried out in DESs as well [35,36] and this can be often connected to microheterogeneity, suggestions for DESs which exhibit strong MH are desirable.MH studies in DES started when hydrophobic DES were presented and successfully applied in extraction processes. [17,37] A nanoscale spatial heterogeneity was found from molecular dynamics simulations of acetamide with Li þ ClO À 4 and propionamide with Li þ ClO À 4 .…”

[Hg4Te8(Te2)4]8−: A Heavy Metal Porphyrinoid Embedded in a Lamellar Structure

“…Besides the segregation of such cations into polar/nonpolar sections in the solid state, [17] similar effects of nanoscale segregation into microphases of homogeneous polarity were also observed in the liquid state of ionic liquids and confirmed by means of molecular dynamic simulations. [18,19] External diameters of the anion range from 12.3 (Te1···Te1')t o16.2 (Te7···Te7'). [36] Thef ormation of heavy homologues of well-known molecules is af ascinating area of chemical research, which gives information about the correlation between intrinsic atomic properties and the structural, chemical, and physical properties of molecules.P rominent examples are heavy ethene and ethyne homologues,w ith their typical impact on their molecular structures.…”