Aggregation Behavior of Several Ionic Liquids in Molecular Solvents of Low Polarity—Indication of a Bimodal Distribution

Abstract

The structure and dynamics of ion pairing and aggregation is studied by concentration- and temperature-dependent measurements of (1) H and (19) F self-diffusion coefficients, viscosity, and conductivity for the following five solutions: 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Cn mim][NTf2], n=2, 4, 6) in dichloromethane (CH2Cl2), and [C6 mim][NTf2] in tetrahydrofuran (THF) and chlorobenzene (C6 H5 Cl). The temperature dependence of these properties at constant IL concentrations follows the Arrhenius law for all five solutions. The IL-concentration dependence of the respective activation energies obtained from the Arrhenius analysis is nonlinear in the case of conductivity, but indicates linear relationships for viscosity and self-diffusion. All five solutions studied display average solute radius maxima as plotted against IL concentration. The maximum average solute radii follow an order of solvents of CHCl3 >C6 H5 Cl>CH2 Cl2 ≈THF, which corresponds to the order of increasing solvent dielectric constant. The observed trends in the physical properties of these solutions indicate the development of a bimodal distribution of solute size with increasing IL concentration. Specifically, the presence of aggregates is supported by the analysis of the conductivity data and the observation of the same self-diffusion coefficients for the cation and anion. The concurrent presence of freely dissolved ions is supported by the obtained average solute radii that do not exceed the radii of the corresponding ion pairs.

Combining Freezing Point Depression and Self-Diffusion Data for Characterizing Aggregation

“…A similar observation of a maximum of r has recently been observed for the aggregation behavior of several imidazolium based ionic liquids in solvents of low polarity. − The maximum has been interpreted with a change in the transport mechanism. − Specifically, at low concentrations, mass transport is reflected by the movement of individual solute molecules and aggregates, while at high concentrations the movement by the large aggregates becomes negligibly small and mass transport is dominated by individual molecules and small aggregates moving from large aggregate to large aggregate. This viewpoint can be further illustrated considering that D C10E6 is the mole fraction weighted average of the self-diffusion coefficient of each present C 10 E 6 species as expressed in eq for the simplifying case of considering only unaggregated C 10 E 6 ( x free ) and aggregated C 10 E 6 ( x agg ): Using eq , eq can be re-expressed in terms of corresponding species radii as shown in eq . From eq , it can be seen that, as long as there is a significant concentration of unaggregated C 10 E 6 , the “free” C 10 E 6 term in eq will dominate for r agg ≫ r free .…”

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

“…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 .…”

Transport Properties of the 1-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide–Trichloromethane Binary System: Indication of Trichloromethane Segregation

“…59 Similar radii maxima were observed in a subsequent study of five other [C n=2,4,6 mim][NTf 2 ]−molecular solvent systems. 60 The radii maxima were interpreted as indicative of a qualitative change in IL transport mechanism from being dominated by individual species moving on their own through the molecular solvent medium to being dominated by individual ions and ion pairs jumping from aggregate to aggregate at higher concentrations. 59,60 One major goal of this study is to inspect if there are any additional changes in the speciation and dynamics in the [C 6 mim][NTf 2 ]−CHCl 3 binary system for the compositions 0.1 < x IL < 1.…”

“…60 The radii maxima were interpreted as indicative of a qualitative change in IL transport mechanism from being dominated by individual species moving on their own through the molecular solvent medium to being dominated by individual ions and ion pairs jumping from aggregate to aggregate at higher concentrations. 59,60 One major goal of this study is to inspect if there are any additional changes in the speciation and dynamics in the [C 6 mim][NTf 2 ]−CHCl 3 binary system for the compositions 0.1 < x IL < 1. For this purpose, self-diffusion coefficients of all three species, cation, anion, and CHCl 3 , were investigated and correlated with conductivity measurements to assess in how far mass transport is carried by neutral or charged species.…”