Towards an Understanding of the Mutual Solubilities of Water and Hydrophobic Ionic Liquids in the Presence of Salts: The Anion Effect

Abstract

The understanding of the specific interactions between salt ions and ionic liquids (ILs) in aqueous solutions is relevant in multiple applications. The influence of a series of anions on the solubility of 1-butyl-3-methylimidazolium tricyanomethane in aqueous environment was here studied. This study aims at gathering further information to evaluate the recently proposed 1,2 mechanisms of salting-in-and salting-out-inducing ions in aqueous solutions of ILs and to provide insights at the molecular-level on the phenomena occurring in these systems. The observed effect of the inorganic species on the aqueous solubility of the ionic liquid qualitatively follows the Hofmeister series, and it is dependent on the nature and concentration of the anions. The liquid-liquid equilibrium data and 1 H NMR results here reported support a model according to which salting-in-and salting-out-inducing ions operate by essentially different mechanisms. While salting-out is an entropically driven effect resulting from the formation of hydration complexes and the increase of the surface tension of cavity formation, the salting-in phenomena is a consequence of the direct binding of the ions to the hydrophobic moieties of the IL. Further evidence here obtained suggests that the interactions of the inorganic ions are not only established with the cation of the IL, but also with the anion, with the observed solubility effect the result of a balance between those two types of interactions.

Salting-in with a Salting-out Agent: Explaining the Cation Specific Effects on the Aqueous Solubility of Amino Acids

“…15,16,31,32 One of the most consistent theories was suggested by Zhang and co-workers 9,17,18 who described ion specific effects on the solubilities of macromolecules in terms of direct interactions of the ions with the solutes and with water molecules. This theory formed in fact a basis for the model that we have been developing and refining to interpret the solubility of charged molecules in aqueous solutions of inorganic salts or amino acids [19][20][21][22]24 and to explain the behavior of aqueous saline solutions of amino acids, 23 which is consistent with the most recent theories that underline the central role of ionic polarizabilities and of ion size in the interpretation of Hofmeister effects. 31,32 Despite a century of efforts, however, in an era characterized by profound technological and scientific advances, the knowledge of the molecular level interactions which govern the behavior of biomolecules in aqueous saline environments is still elusive.…”

Ionic liquids in enhancing the sensitivity of capillary electrophoresis: Off‐line and on‐line sample preconcentration techniques

“…of water-ion complexes whereas salting-in inducing ions directly interact with the hydrophobic moieties of the IL [40][41][42]. The salting-in/-out effects are thus dominated by the ion-ion versus ion-water interactions and not by underlying water structure modifications, as classically accepted.…”

Separating Rare-Earth Elements with Ionic Liquids

“…Moreover, the salting-out effect can be used to enhance an IL during an aqueous-phase separation [77]. The influence of inorganic salts on the solubility of [C 4 mim] [tricyanomethane] in aqueous solutions has been widely documented and has been used to derive a salting-in and salting-out mechanism [78]. It has been noted that interactions between the anionic species of an inorganic salt not only influence the cationic species of an IL but the anionic species as well [78].…”

“…The influence of inorganic salts on the solubility of [C 4 mim] [tricyanomethane] in aqueous solutions has been widely documented and has been used to derive a salting-in and salting-out mechanism [78]. It has been noted that interactions between the anionic species of an inorganic salt not only influence the cationic species of an IL but the anionic species as well [78]. The observed solubility of an IL in an aqueous solution containing an inorganic salt is a direct consequence of a balance between these two kinds of interactions with the anionic species of the inorganic salt in aqueous solution [78].…”

“…It has been noted that interactions between the anionic species of an inorganic salt not only influence the cationic species of an IL but the anionic species as well [78]. The observed solubility of an IL in an aqueous solution containing an inorganic salt is a direct consequence of a balance between these two kinds of interactions with the anionic species of the inorganic salt in aqueous solution [78].…”