Ion Specific Effects on the Mutual Solubilities of Water and Hydrophobic Ionic Liquids

Abstract

Ion specific effects on the mutual solubilities between hydrophobic ionic liquids (ILs) and water are complex and not fully understood. The aim of this work is to obtain further evidence about the molecular mechanism behind this phenomenon by evaluating the effect of a large series of inorganic and organic salts on the mutual solubilities of water and the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(4)mim][Tf(2)N]. The magnitudes of the salting-in and salting-out effects were assessed by changing either the cation or the anion, in a series of salts, as well as the salt concentration. It was observed that the influence of the ions on the solubility followed the Hofmeister series. Both salting-in and salting-out effects were observed and they showed to be dependent on both the nature of the salt and its concentration, while the pH had only a marginal effect on the studied solubilities. On the basis of the solubility changes of the ionic liquid in water in the presence of salts and on NMR spectroscopic data, it will be shown that salting-out inducing ions (high charge density) and salting-in inducing ions (low charge density) act through different mechanisms. While the former act mainly through an entropic effect resulting from the formation of water-ion hydration complexes which cause the dehydration of the solute and the increase of the surface tension of the cavity, the salting-in results from a direct ion binding of the low charge density ions to the hydrophobic moieties of the solute.

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

“…[40] Our present data fit well also with the data published by the group of Coutinho [29] when ultrapure water was used at 25 8C: [20] Our value (corresponding to 0.7 wt % of C 1 C 4 im + in H 2 O) is also in fair agreement with the C 1 C 4 im + data of Freire and co-workers, [42] lost to the aqueous phase at maximum. This effect of sodium perchlorate addition is also observed for n = 2, n = 6, and n = 8, but it stabilizes at % 20 mm for n = 10.…”

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

“…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].…”

Enzymatic activity and thermal stability of metallo proteins in hydrated ionic liquids

“…In general in the aqueous phase, a pair of strong kosmotropic anions, and a chaotrope cation, stabilize proteins, whereas strong kosmotropic cations destabilize proteins. [15][16][17] Even in ILs, there have recently been many reports about the effect of kosmotro-picity on physical properties such as two phase behavior, 18 and on the activity of biomolecules. 19 In our previous report we found that the effect of component ions on cyt c was correlated with kosmotropicity.…”