The influence of hydrogen bonding on the physical properties of ionic liquids

Abstract

Potential applications of ionic liquids depend on the properties of this class of liquid material. To a large extent the structure and properties of these Coulomb systems are determined by the intermolecular interactions among anions and cations. In particular the subtle balance between Coulomb forces, hydrogen bonds and dispersion forces is of great importance for the understanding of ionic liquids. The purpose of the present paper is to answer three questions: Do hydrogen bonds exist in these Coulomb fluids? To what extent do hydrogen bonds contribute to the overall interaction between anions and cations? And finally, are hydrogen bonds important for the physical properties of ionic liquids? All these questions are addressed by using a suitable combination of experimental and theoretical methods including newly synthesized imidazolium-based ionic liquids, far infrared spectroscopy, terahertz spectroscopy, DFT calculations, differential scanning calorimetry (DSC), viscometry and quartz-crystal-microbalance measurements. The key statement is that although ionic liquids consist solely of anions and cations and Coulomb forces are the dominating interaction, local and directional interaction such as hydrogen bonding has significant influence on the structure and properties of ionic liquids. This is demonstrated for the case of melting points, viscosities and enthalpies of vaporization. As a consequence, a variety of important properties can be tuned towards a larger working temperature range, finally expanding the range of potential applications.

Local structures of ionic liquids in the presence of gold under high pressures

“…3 understanding of structure formation in ionic liquids and extended hydrogen-bond networks has been reported for pure ionic liquids. [1][2][3][4][5] It is known that the imidazolium C-H stretching absorptions may be described by four bands (two doublets), one doublet at the lower frequency is assigned to the C 2 -H stretching mode and another doublet at the higher frequency is attributed to the coupled 4 -H and C 5 -H stretching modes. 33 The nearly degenerated imidazolium C-H features at ca.3100 and 3118 cm −1 are related to hydrogen bonding structures and microheterogenity.…”

“…• C. [1][2][3][4][5][6][7][8] Ionic liquids have attracted more and more attention owing to their dual nature as both salts and fluids. Furthermore, ionic liquids have peculiar properties like negligible vapor pressure, wide liquid range, high electrical conductivity, and tunable miscibility.…”

Azolium-based systems: application of an anion exchange resin (A- form) method and 1H NMR analysis of the charged-assisted (C–H)+•anion hydrogen bonds

“…36 Moreover, Ludwig and co-workers reported direct spectroscopic evidence for an enhanced cation-anion interaction driven by (C-H) + ···anion hydrogen bonds in pure ILs, which gave a stronger the role to hydrogen bonds in imidazolium ILs. [19][20] Depending on the structure of the imidazolium-based frameworks, other noncovalent intermolecular interactions can also take place between cations and counteranions, a case in point being the usually weaker CH/π noncovalent interactions that can be rather significant for anions bearing aromatic units, e.g. tetraphenylborate.…”

“…It should be noted that in the last years theoretical calculations have confronted the question of what is responsible for the anion−cation non-covalent interactions in pure imidazolium-based ILs and have challenged the role of (C−H) + hydrogen bonds in explaining IL properties. 19,20 A relevant part of the present study is focused on the significance of the noncovalent interactions involved between the azolium motifs and a variety of anions, with special attention given to nonclassical charged-assisted (C-H) + ···anion hydrogen bonds. Thus, the ion pairs prepared provided the opportunity to learn about the hydrogen-bonding interactions of simple azolium systems such as 1-butyl-3-methylimidazolium 1a·A ([bmim]A) and 1-benzyl-1-methyl-1,2,3-triazolium 2b·A ([BnmTr]A) salts in solution-phase by proton NMR spectroscopy.…”

Doubly ionic hydrogen bond interactions within the choline chloride–urea deep eutectic solvent

“…8 ' 9, 10 H-bonding is 50 recognised to be one of the key interactions responsible for influencing the structuring and properties of ILs. [11][12][13][14][15][16][17] Due to the vast number of possible ion combinations, the extent of Hbonding in ILs is highly system dependent. The potential to modulate the relative contributions of electrostatics, dispersion 55 and H-bonding through ion design, and thus influence a range of physico-chemical properties, is one of the desirable attributes of ILs.…”