Developing design tools for introducing and tuning structural order in ionic liquids

Abstract

Ionic liquids (ILs) are receiving growing interest as highly tunable, multifunctional materials. Remarkably for liquids, they tend to display a high level of structural order. This structural order may even lead to the formation of mesophases such as liquid crystals (LCs). Imidazolium compounds are by far the most popular ILs, because they offer a widely versatile platform for property tuning. To investigate what is driving structural order in imidazolium-based ILs a series of asymmetrical 1-dodecyl-2-methyl-3-alkylimidazolium bromides, [C₁₂C₁C_nim][Br] with n = 0–12 have been synthesized, fully characterized and their structures and properties compared with the analogous 1-dodecyl-3-alkylimidazolium as well as the 1,2,3-triazolium bromides. The aim is to examine the influence of the replacement of the most acidic 2-H proton on the imidazolium head group by methylation on the properties and structure of ILs. For all compounds, except for compounds with butyl- and hexyl-chains as well as the protonated species, mesophase formation can be observed. Obviously, the simple presence of long alkyl chains such as dodecyl (a design concept frequently put forward in the literature) is not sufficient to support mesophase formation alone. Rather, for the formation of a liquid crystalline phase, a balance between attractive van der Waals forces, hydrogen bonds, and electrostatic interactions is required. Data from temperature-dependent small-angle X-ray scattering (SAXS) and polarizing optical microscopy (POM) suggest three different cation conformations for the studied [C₁₂C₁C_nim][Br]: cations with 0 ≤ n ≤ 4 exhibit a near-linear conformation; for 5 ≤ n ≤ 10 a V-shape is adopted, and for n = 11 or 12 a U-shape is found. We demonstrated that the structural possibility for an interdigitation of the long chains is an influential factor for the formation of a mesophase.