Abstract
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][CnF2n+1SO3], [C2C1im][CnH2n+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 [N1 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.
Highlights
Ionic liquids (ILs) are salt-like ionic materials with unusually low melting point temperatures (100 ◦ C being the usually accepted threshold) [1]
As for ethers, it is one of the few systems with ILs that exhibits phase diagrams characterized by the existence of a lower critical solution temperature—a phenomenon that is related to the breaking-up of the hydrogen-bonded network that can be formed between the functionalized cation of the ionic liquid and the oxygen atom of the ether molecule [32]
The focus will be on aqueous solutions of diverse ionic liquids (Chart 1), namely those containing (a) different cations with small alkyl sidechains; (b) different anions based on sulfonate or carboxylate moieties attached to hydrogenated or fluorinated alkyl chains; and (c) cations with high hydrogen-bonding capabilities
Summary
Ionic liquids (ILs) are salt-like ionic materials with unusually low melting point temperatures (100 ◦ C being the usually accepted threshold) [1]. Introducing atoms with lone pairs of electrons capable of accepting a proton and establishing a hydrogen bond with amino-based alcohols adds further possible combinations in which interaction is possible with the IL [31] As for ethers, it is one of the few systems with ILs that exhibits phase diagrams characterized by the existence of a lower critical solution temperature—a phenomenon that is related to the breaking-up of the hydrogen-bonded network that can be formed between the functionalized cation of the ionic liquid and the oxygen atom of the ether molecule [32]. The focus will be on aqueous solutions of diverse ionic liquids (Chart 1), namely those containing (a) different cations with small alkyl sidechains (including branched ones); (b) different anions based on sulfonate or carboxylate moieties attached to hydrogenated or fluorinated alkyl chains; and (c) cations with high hydrogen-bonding capabilities. Molecular structure and acronyms of the ions composing the ionic liquids studied in this work
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