Ionic Liquids and Water: Hydrophobicity vs. Hydrophilicity

Many chemical processes rely extensively on organic solvents posing safety and environmental concerns. For a successful transfer of some of those chemical processes and reactions to aqueous media, agents acting as solubilizers, or phase-modifiers, are of central importance. In the present work, the structure of aqueous solutions of several ionic liquid systems capable of forming multiple solubilizing environments were modeled by molecular dynamics simulations. The effect of small aliphatic chains on solutions of hydrophobic 1-alkyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide ionic liquids (with alkyl = propyl [C₃C₁im][NTf₂], butyl [C₄C₁im][NTf₂] and isobutyl [iC₄C₁im][NTf₂]) are covered first. Next, we focus on the interactions of sulphonate- and carboxylate-based anions with different hydrogenated and perfluorinated alkyl side chains in solutions of [C₂C₁im][C_nF_2n+1SO₃], [C₂C₁im][C_nH_2n+1SO₃], [C₂C₁im][CF₃CO₂] and [C₂C₁im][CH₃CO₂] (n = 1, 4, 8). The last system considered is an ionic liquid completely miscible with water that combines the cation N-methyl-N,N,N-tris(2-hydroxyethyl)ammonium [N_{1 2OH 2OH 2OH}]⁺, with high hydrogen-bonding capability, and the hydrophobic anion [NTf₂]^-. The interplay between short- and long-range interactions, clustering of alkyl and perfluoroalkyl tails, and hydrogen bonding enables a wealth of possibilities in tailoring an ionic liquid solution according to the needs.

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems

This work highlights unexpected, not so well known responses of ionic liquids and ionic liquid-containing systems, which are reported in a collective manner, as a short review. Examples include: (i) Minima in the temperature dependence of the isobaric thermal expansion coefficient of some ILs; (ii) Viscosity Minima in binary mixtures of IL + Molecular solvents; (iii) Anomalies in the surface tension within a family of ILs; (iv) The constancy among IL substitution of C_p/V_m at and around room temperature; (v) ILs as glass forming liquids; (vi) Alternate odd-even side alkyl chain length effects; (vii) Absolute negative pressures in ILs and IL-containing systems; (viii) Reversed-charged ionic liquid pairs; (ix) LCST immiscibility behavior in IL + solvent systems.

"Solvent-in-salt" systems for design of new materials in chemistry, biology and energy research

Inorganic and organic "solvent-in-salt" (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

Designing the ammonium cation to achieve a higher hydrophilicity of bistriflimide-based ionic liquids

Development of water soluble bistriflimide-based ionic liquids and molecular dynamics simulations to unveil the underlying molecular details.

Solvent effects on the polar network of ionic liquid solutions

Molecular dynamics simulations were used to probe mixtures of ionic liquids (ILs) with common molecular solvents. Four types of systems were considered: (i) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide plus benzene, hexafluorobenzene or 1,2-difluorobenzene mixtures; (ii) choline-based ILs plus ether mixtures (iii) choline-based ILs plus n-alkanol mixtures; and (iv) 1-butyl-3-methylimidazolium nitrate and 1-ethyl-3-methylimidazolium ethyl sulfate aqueous mixtures. The results produced a wealth of structural and aggregation information that highlight the resilience of the polar network of the ILs (formed by clusters of alternating ions and counter-ions) to the addition of different types of molecular solvent. The analysis of the MD data also shows that the intricate balance between different types of interaction (electrostatic, van der Waals, H-bond-like) between the different species present in the mixtures has a profound effect on the morphology of the mixtures at a mesoscopic scale. In the case of the IL aqueous solutions, the present results suggest an alternative interpretation for very recently published x-ray and neutron diffraction data on similar systems.

Effects of constituent ions of a phosphonium-based ionic liquid on molecular organization of H2O as probed by 1-propanol: tetrabutylphosphonium and trifluoroacetate ions

The phosphonium-based cation, [P₄₄₄₄]⁺, is significant amphiphile with strong hydrophobic and equally strong hydrophilic contributions.

Thermoresponsive polyelectrolytes derived from ionic liquids

In this review we summarise recent progress on the design, properties, and potential applications of ionic liquid-derived polyelectrolytes showing thermoresponsive phase behaviour after mixing with water or other organic solvents.

Homogeneous liquid–liquid extraction of metal ions with non-fluorinated bis(2-ethylhexyl)phosphate ionic liquids having a lower critical solution temperature in combination with water

Bis(2-ethylhexyl)phosphate ionic liquids show a temperature-dependent phase behaviour of the LCST-type and can extract transition metal ions very efficiently via homogeneous liquid–liquid extraction.

Spectroscopic evidence of ‘jumping and pecking’ of cholinium and H-bond enhanced cation–cation interaction in ionic liquids

Spectroscopic evidence for cation–cation interaction in ionic liquids. The repulsive electrostatic interaction is overcome by hydrogen bonding between ions of like charge.

Preparation and thermoresponsive properties of helical polypeptides bearing pyridinium salts

Polypeptides bearing 3-methylpyridinium groups and BF₄⁻ prepared by nuleophilic substitution and ion-exchange reaction showed upper critical solution temperature (UCST)-type transitions in aqueous solutions.

The complex structure of ionic liquids at an atomistic level: from “red-and-greens” to charge templates

In this article, we discuss how the relation between interactions and structure in ionic liquids (ILs) can be probed at a molecular level using ab initio and molecular dynamics (MD) methodologies. The first part of the discussion will focus on the unique and complex properties of ILs as pure substances including the existence of an extended and flexible polar network and the possibility of a second nanosegregated subphase containing the nonpolar residues of the molecular ions that constitute some ILs. The discussion will then be extended to IL plus molecular species mixtures/solutions. In this context the concept of ILs as charge templates for the electronic make-up of the molecular species will be analyzed at length. Finally, that concept will be extended to ILs adsorbed over solid substrates.