Tuning Binary Ionic Liquid Mixtures: Linking Alkyl Chain Length to Phase Behavior and Ionic Conductivity

The use of mixed salts to generate new composite ionic liquids (ILs) provides a facile means of readily tuning or tailoring the desired properties of ionic media. Despite this, very little information is available about how the structure of the selected ions and composition impacts the properties of salt mixtures. To explore this, six binary IL₁–IL₂ mixtures based on <i>N</i>-alkyl-<i>N</i>-methylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide salts have been characterized. The physicochemical properties (density, viscosity, and ionic conductivity) and phase behavior of these mixtures are reported. The variation of the alkyl chains lengths on the cations plays a significant role in determining both the phase behavior and the physicochemical properties of the mixtures. Notably, the “tunability” of the properties of the IL mixtures is much easier to control than is found by simply making small structural changes to the ions in a given salt

Physicochemical Properties of Binary Ionic Liquid–Aprotic Solvent Electrolyte Mixtures

The properties of mixtures of ionic liquids (ILs) with a variety of different aprotic solvents have been examined in detail. The ILs selectedbis­(trifluoromethanesulfonyl)­imide (TFSI^–) salts with <i>N</i>-methyl-<i>N</i>-pentylpyrrolidinium (PY₁₅⁺), -piperidinium (PI₁₅⁺), or -morpholinium (MO₁₅⁺) cationsenabled the investigation of how cation structure influences the mixture properties. This study includes the characterization of the thermal phase behavior of the mixtures and volatility of the solvents, density and excess molar volume, and transport properties (viscosity and conductivity). The mixtures with ethylene carbonate form a simple eutectic, whereas those with ethyl butyrate appear to form a new IL–solvent crystalline phase. Significant differences in the viscosity of the mixtures are found for different solvents, especially for the IL-rich concentrations. In contrast, only minor differences are noted for the conductivity with different solvents for the IL-rich concentrations. For the solvent-rich concentrations, however, substantial differences are noted in the conductivity, especially for the mixtures with acetonitrile

Influence of Solvent on Ion Aggregation and Transport in PY₁₅TFSI Ionic Liquid–Aprotic Solvent Mixtures

Molecular dynamics (MD) simulations using a many-body polarizable APPLE&P force field have been performed on mixtures of the <i>N</i>-methyl-<i>N</i>-pentylpyrrolidinium bis­(trifluoromethanesulfonyl)­imide (PY₁₅TFSI) ionic liquid (IL) with three molecular solvents: propylene carbonate (PC), dimethyl carbonate (DMC), and acetonitrile (AN). The MD simulations predict density, viscosity, and ionic conductivity values that agree well with the experimental results. In the solvent-rich regime, the ionic conductivity of the PY₁₅TFSI–AN mixtures was found to be significantly higher than the conductivity of the corresponding −PC and −DMC mixtures, despite the similar viscosity values obtained from both the MD simulations and experiments for the −DMC and −AN mixtures. The significantly lower conductivity of the PY₁₅TFSI–DMC mixtures, as compared to those for PY₁₅TFSI–AN, in the solvent-rich regime was attributed to the more extensive ion aggregation observed for the −DMC mixtures. The PY₁₅TFSI–DMC mixtures present an interesting case where the addition of the organic solvent to the IL results in an increase in the cation–anion correlations, in contrast to what is found for the mixtures with PC and AN, where ion motion became increasingly uncorrelated with addition of solvent. A combination of pfg-NMR and conductivity measurements confirmed the MD simulation predictions. Further insight into the molecular interactions and properties was also obtained using the MD simulations by examining the solvent distribution in the IL–solvent mixtures and the mixture excess properties

Diffusion Coefficients from ¹³C PGSE NMR MeasurementsFluorine-Free Ionic Liquids with the DCTA^– Anion

Pulsed-field gradient spin–echo (PGSE) NMR is a widely used method for the determination of molecular and ionic self-diffusion coefficients. The analysis has thus far been limited largely to ¹H, ⁷Li, ¹⁹F, and ³¹P nuclei. This limitation handicaps the analysis of materials without these nuclei or for which these nuclei are insufficient for complete characterization. This is demonstrated with a class of ionic liquids (or ILs) based on the nonfluorinated anion 4,5-dicarbonitrile-1,2,3-triazole (DCTA^–). It is demonstrated here that ¹³C-PGSE NMR can be used to both verify the diffusion coefficients obtained from other nuclei, as well as characterize materials that lack commonly scrutinized nuclei  all without the need for specialized NMR methods