Competition between Cation–Solvent and Cation–Anion Interactions in Imidazolium Ionic Liquids with Polar Aprotic Solvents

Abstract

The subtle interplay between ion solvation and association was analyzed in mixtures of imidazolium-based ionic liquids (ILs) with polar aprotic solvents. A site-specific pattern of cation-solvent and cation-anion interactions was disclosed by a careful analysis of the H and C NMR chemical shift dependence of the mixture composition. It was established that the less polar but more donating γ-butyrolactone is more prone to establish H-bonds with the imidazolium-ring hydrogen atoms of the IL cations than propylene carbonate, particularly at the H site and at high dilutions x <0.1. The H site was found to be more sensitive to intermolecular interactions compared to H in the case of ILs with asymmetric anions like trifluoromethanesulfonate (TfO ) or bis(trifluoromethylsulfonyl)amide (TFSA ).

Computational NMR Study of Ion Pairing of 1-Decyl-3-methyl-imidazolium Chloride in Molecular Solvents

“… 37 Even though acetonitrile is rather similar to DMSO in polarity, it does not seem to break ionic aggregates into separate ions, apparently due to the poorer hydrogen-bond accepting abilities. 45 , 48 − 50 Varying degrees of dissociation of the [C4mim]-based 35 , 51 , 52 and ammonium-based 41 ILs in acetonitrile solution were, however, reported. At low concentrations in nonpolar solvents, ILs tend to form supramolecular aggregates, 35 , 44 , 51 , 53 − 55 although lone contact ion pairs were seen to be the dominant association pattern for some ILs in chloroform 47 and dichloromethane.…”

“…Indeed, the low-concentration aqueous solutions of ILs behave as traditional electrolytes, where ions are virtually completely solvated, ,− although in some instances ion pairing was detected even at rather low concentrations of the IL. , Dimethyl sulfoxide, DMSO, has also demonstrated the capability of breaking ionic aggregates into separate ions, ,− but, here too, some ILs have displayed slight association at high dilutions. , Remarkably, however, higher-order supramolecular ionic clusters were found to persist at rather low concentrations of [C4mim]­[Cl] IL in DMSO . Even though acetonitrile is rather similar to DMSO in polarity, it does not seem to break ionic aggregates into separate ions, apparently due to the poorer hydrogen-bond accepting abilities. ,− Varying degrees of dissociation of the [C4mim]-based ,, and ammonium-based ILs in acetonitrile solution were, however, reported. At low concentrations in nonpolar solvents, ILs tend to form supramolecular aggregates, ,,,− although lone contact ion pairs were seen to be the dominant association pattern for some ILs in chloroform and dichloromethane. , However, the dissociation of [C4mim]­[Cl] ion pairs into separate ions in solutions of chloroform and carbon tetrachloride has been observed very recently. , …”

“…Nuclear magnetic resonance, NMR, spectroscopy has proven to be a useful tool in structural and dynamical investigations of IL systems. ,,− For the imidazolium family of ILs, the chemical shift of the H2 proton has been recognized to be a sensitive probe of the local environment of the imidazolium cation ,,,,, because the C2–H2 bond is the most acidic bond in the imidazolium ring and thus is typically the most favorable site for hydrogen bonding. , In one particular case, the NMR signal of the H2 proton in the solutions of [C10mim]­[Br] IL has displayed a rather strong solvent dependence as it was found to be the highest at about 10.6 ppm in dichloromethane, DCM, and the lowest at about 8.9 ppm in water with an intermediate chemical shift at around 10.3 ppm recorded in acetonitrile, ACN, all w.r.t. Si­(CH 3 ) 4 .…”

Cation–Anion Interactions in 1-Ethyl-3-methylimidazolium-Based Ionic Liquids with Aprotic Heterocyclic Anions (AHAs)

“…Marekha et al investigated the effects of aprotic polar solvents, such as propylene carbonate and γ-butyrolactone, on imidazolium based ILs by measuring variations in NMR chemical shifts. They observed a decrease in the strength of the interionic interactions in the presence of those solvents . Additionally, computational studies have shown that generally stronger hydrogen bonding is observed in IL systems with polar and protic solvents than with aprotic solvents. , Thus, there have been many spectroscopic and computational studies of interactions for imidazolium based ILs, but these studies primarily focused on ILs with very weakly basic anions that do not have the ability to chemically react with CO 2 .…”

Insights on the mixtures of imidazolium based ionic liquids with molecular solvents

“…These characteristics determine the properties and structure of pure ILs but also have a large effect when mixing with molecular solvents (MSs) [4][5][6][7]. Intermolecular forces between ILs and MSs are largely complex [810], even with the appearance of competition effects between the involved ions and molecules [11], because of their dependence on the protic or aprotic, polar or apolar, or hydrogen bonding ability of the MSs, and show remarkable changes with mixture composition, pressure and temperature [12][13][14][15]. These mechanisms of ILs-MSs interactions determine the macroscopic physicochemical properties of mixed fluids [1618], and thus, their possible application for several technologies at industrial level [1921].…”