Ionic Liquids: Dissecting the Enthalpies of Vaporization

We calculate the heats of vaporisation for imidazolium‐based ionic liquids [C_nmim][NTf₂] with n=1, 2, 4, 6, 8 by means of molecular dynamics (MD) simulations and discuss their behavior with respect to temperature and the alkyl chain length. We use a force field developed recently. The different cohesive energies contributing to the overall heats of vaporisations are discussed in detail. With increasing alkyl chain length, the Coulomb contribution to the heat of vaporisation remains constant at around 80 kJ mol^?1, whereas the van der Waals interaction increases continuously. The calculated increase of about 4.7 kJ mol^?1 per CH₂‐group of the van der Waals contribution in the ionic liquid exactly coincides with the increase in the heats of vaporisation for n‐alcohols and n‐alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions.     

Imidazolium Based Ionic Liquids: Unbiased Recovering of Vaporization Enthalpies from Infinite-Dilution Activity Coefficients

We propose and test an efficient approach for the assessment of the enthalpies of vaporization of ionic liquids at the reference temperature 298.15 K. The approach is based on activity coefficients at infinite dilution of volatile organic solutes in ionic liquids bearing the imidazolium cation of the general formula [C_nmim][Anion].     

X‐ray Photoelectron Spectroscopy of Pyridinium‐Based Ionic Liquids: Comparison to Imidazolium‐ and Pyrrolidinium‐Based Analogues

We investigate eight 1‐alkylpyridinium‐based ionic liquids of the form [C_nPy][A] by using X‐ray photoelectron spectroscopy (XPS). The electronic environment of each element of the ionic liquids is analyzed. In particular, a reliable fitting model is developed for the C 1s region that applies to each of the ionic liquids. This model allows the accurate charge correction of binding energies and the determination of reliable and reproducible binding energies for each ionic liquid. Shake‐up/off phenomena are determinedfor both C 1s and N 1s spectra. The electronic interaction between cations and anions is investigated for both simple ionic liquids and an example of an ionic‐liquid mixture; the effect of the anion on the electronic environment of the cation is also explored. Throughout the study, a detailed comparison is made between [C₈Py][A] and analogues including 1‐octyl‐1‐methylpyrrolidinium‐ ([C₈C₁Pyrr][A]), and 1‐octyl‐3‐methylimidazolium‐ ([C₈C₁Im][A]) based samples, where X is common to all ionic liquids.     

Ion Pairing in Protic Ionic Liquids Probed by Far‐Infrared Spectroscopy: Effects of Solvent Polarity and Temperature

The cation–anion and cation–solvent interactions in solutions of the protic ionic liquid (PIL) [Et₃NH][I] dissolved in solvents of different polarities are studied by means of far infrared vibrational (FIR) spectroscopy and density functional theory (DFT) calculations. The dissociation of contact ion pairs (CIPs) and the resulting formation of solvent‐separated ion pairs (SIPs) can be observed and analyzed as a function of solvent concentration, solvent polarity, and temperature. In apolar environments, the CIPs dominate for all solvent concentrations and temperatures. At high concentrations of polar solvents, SIPs are favored over CIPs. For these PIL/solvent mixtures, CIPs are reformed by increasing the temperature due to the reduced polarity of the solvent. Overall, this approach provides equilibrium constants, free energies, enthalpies, and entropies for ion‐pair formation in trialkylammonium‐containing PILs. These results have important implications for the understanding of solvation chemistry and the reactivity of ionic liquids.     

Recombination of Photogenerated Lophyl Radicals in Imidazolium‐Based Ionic Liquids

Laser flash photolysis is applied to study the recombination reaction of lophyl radicals in ionic liquids in comparison with dimethylsulfoxide as an example of a traditional organic solvent. The latter exhibits a similar micropolarity as the ionic liquids. The ionic liquids investigated are 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ( 1 ), 1‐hexyl‐3‐methylimidazolium hexafluorophosphate ( 2 ), and 1‐butyl‐3‐methylimidazolium tetafluoroborate ( 3 ). The recombination of the photolytic generated lophyl radicals occur significantly faster in the ionic liquids than expected from their macroscopic viscosities and is a specific effect of these ionic liquids. On the other hand, this reaction can be compared with the macroscopic viscosity in the case of dimethylsulfoxide. Activation parameters obtained for lophyl radical recombination suggest different, anion‐dependent mechanistic effects. Quantum chemical calculations based on density functional theory provide a deeper insight of the molecular properties of the lophyl radical and its precursor. Thus, excitation energies, spin densities, molar volumes, and partial charges are calculated. Calculations show a spread of spin density over the three carbon atoms of the imidazolyl moiety, while only low spin density is calculated for the nitrogens.     

Dispersion and Hydrogen Bonding Rule: Why the Vaporization Enthalpies of Aprotic Ionic Liquids Are Significantly Larger than those of Protic Ionic liquids

It is well known that gas‐phase experiments and computational methods point to the dominance of dispersion forces in the molecular association of hydrocarbons. Estimates or even quantification of these weak forces are complicated due to solvent effects in solution. The dissection of interaction energies and quantification of dispersion interactions is particularly challenging for polar systems such as ionic liquids (ILs) which are characterized by a subtle balance between Coulomb interactions, hydrogen bonding, and dispersion forces. Here, we have used vaporization enthalpies, far‐infrared spectroscopy, and dispersion‐corrected calculations to dissect the interaction energies between cations and anions in aprotic (AILs), and protic (PILs) ionic liquids. It was found that the higher total interaction energy in PILs results from the strong and directional hydrogen bonds between cation and anion, whereas the larger vaporization enthalpies of AILs clearly arise from increasing dispersion forces between ion pairs.     

NMR Investigation of Imidazolium‐Based Ionic Liquids and Their Aqueous Mixtures

¹H and ¹³C NMR spectroscopy is employed to investigate the interaction of water with two imidazolium‐based ionic liquids (ILs), 1‐hexyl‐3‐methylimidazolium bromide ([C₆mim]Br) and 1‐octyl‐3‐methylimidazolium bromide ([C₈mim]Br), at IL concentrations well above the critical aggregation concentration (CAC). The results are compared with those of the neat samples. To this aim, a detailed analysis of the changes in the ¹H chemical shifts, ¹³C relaxation parameters, and 2D ROESY data due to the presence of water is performed. The results for both neat ILs are consistent with a packed structure where head‐to‐head, head‐to‐tail, and tail‐to‐tail contacts occur and where the site of maximal mobility restriction is at the polar head. At the lowest investigated water content, the presence of water influences mainly the environment around the IL polar head, slowing down the motional dynamics of the aromatic ring with respect to the alkyl chain. At higher water contents this difference diminishes, the motional freedom of the whole molecule increasing. The presence of ROESY cross‐peaks between protons in the polar and apolar IL regions, as well as between protons in non‐neighboring alkyl groups, at all investigated water contents suggests that the alkyl tails are not fully segregated in hydrophobic domains, as expected for micelle‐like structures.     

Polarity Behaviour and Specific Interactions of Imidazolium‐Based Ionic Liquids in Ethylene Glycol

The molecular interactions of the ionic liquids (ILs) 1‐butyl‐3‐methylimidazolium tetrafluoroborate [C₄mim][BF₄], 3‐methyl‐1‐octylimidazolium tetrafluoroborate [C₈mim][BF₄] and 1‐butyl‐3‐methylimidazolium octylsulfate [C₄mim][C₈OSO₃] are investigated in ethylene glycol (EG) over the whole mole fraction range using fluorescence (steady‐state and time‐resolved), Fourier transform infrared and nuclear magnetic resonance (NMR) spectroscopy. The cybotactic region surrounding the pyrene fluorescent probe exhibits peculiar characteristics for different ILs in the EG‐rich region. The extent of solute–solvent interactions is assessed by determining the deviations of experimentally observed vibronic band intensity ratios of peak 1 to peak 3 of pyrene fluorescence (I₁/I₃) from a composite I₁/I₃ value obtained using a preferential solvation model. A distinct vibrational frequency shift for various stretching modes of EG (O? H) or ILs (C? H of ring protons, B? F and S?O of anions) indicates specific interactional preferences of EG toward the IL protons/anion. Splitting of the O? H vibration band of EG at 3000–3700 cm^?1 into three separate bands, and analysis of the changes in location and area of these bands as a function of concentration enable precise determination of the effect of ILs on hydrogen bridges of EG. NMR chemical shifts and their deviations from ideality show multiple hydrogen‐bonding interactions of varying strengths between unlike molecules in the mixtures. A comparison of spectroscopic results with thermodynamic properties shows that the mixing microscopic behaviour of the investigated systems is completely different from the macroscopic behaviour, which is primarily governed by the difference in shape, size and nature of the molecules.