¹H NMR and Molecular Dynamics Evidence for an Unexpected Interaction on the Origin of Salting-In/Salting-Out Phenomena

Abstract

By employing 1 H NMR spectroscopy and molecular simulations, we provide an explanation for recent observations that the aqueous solubilities of ionic liquids exhibit salting-out to salting-in regimes upon addition of distinct inorganic salt ions. Using a typical ionic liquid [1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide], we observed the existence of preferential specific interactions between the low electrical charge density ("apolar moiety") parts of the ionic liquid cation and the inorganic salts. These a priori unexpected interactions become increasingly favorable as one moves from salting-out to salting-in effects. More specifically, this interpretation is validated by distinct aqueous solution 1 H NMR data shifts in the ionic liquid cation upon inorganic salt addition. These shifts, which are well noted in the terminal and preterminal hydrogens of the alkyl chain appended to the imidazolium ring, correlate quantitatively with solubility data, both for cases where the nature of inorganic salt is changed, at constant concentration, and for those where the concentration of a given inorganic salt is varied. Molecular simulations have also been performed permitting us to garner a broader picture of the underlying mechanism and structure of this complex solvation phenomenon. These findings can now be profitably used to anticipate solution behavior upon inorganic salt addition well beyond the specificity of the ionic liquid solutions, i.e., for a diversity of distinct solutes differing in chemical nature.

Modulating the Solubilities of Ionic Liquid Components in Aqueous–Ionic Liquid Biphasic Systems: A Q‐NMR Investigation

“…[50] However, comparisons between our experimental data and these MD results should be made with caution for the following reasons: 1) The calculations were performed for an IL concentration of 0.1 m and salt concentrations in the range 0.2-2 m. These concentration values are more or less out of the range of the chemical conditions we investigated.…”

Extraction of vanillin using ionic-liquid-based aqueous two-phase systems

“…Therefore, this work evaluated the extraction ability of several IL-based ATPS for vanillin. All ILs studied are based on the imidazolium cation: 1-ethyl-3-methylimid azolium chloride, [ 19 F NMR spectra and found to be >99 wt% for all samples. The inorganic salt K 3 PO 4 was supplied from Sigma with a purity level >98 wt%.…”

“…Salting-out inducing ions (high charge density ions) have a greater tendency to form hydration complexes, and thus do not interact with vanillin, decreasing therefore the respective partition coefficients. On the other hand, for salting-in inducing ions (low charge density ions) the tendency to form hydration complexes is marginal and thus they tend to stabilize the solutes in solution by specific ion binding to the solute [16,19].…”

“…Since anions are typically more polarizable than cations, due to their more diffuse valence electronic configuration, their hydration is usually stronger than that of cations and, as a result, their saltingout effects are more prominent [16,19]. Thus, due to the stronger ability of anions for salting-out, and particularly of the methylsulfate anion, the presence of additional vanillin does not conduct to favorable interactions between IL anions and the solute.…”

“…Indeed, few years ago, Gutowski et al [12] showed that aqueous solutions of imidazolium-based ILs can form ATPS under the addition of appropriate inorganic salts, such as K 3 PO 4 . Since then, the equilibrium properties of systems comprising ILs, for the development of specific extraction and isolation procedures, have been inves- tigated at our laboratory [9,[13][14][15][16][17][18][19]. It was already shown that ATPS formed by K 3 PO 4 and ILs are extremely advantageous for the partition of several biomolecules yielding larger partition coefficients than those conventionally obtained with polymer-inorganic salts or polymer-polymer ATPS [6,10,13].…”