Local solvent properties of imidazolium-based ionic liquids

Quantification of Ylide Formation in Phosphonium-Based Ionic Liquids Reacted with CO2

Phosphonium-based ionic liquids (ILs) paired with aprotic heterocyclic anions (AHAs) are found to react with CO₂ to form both a carbamate product and a carboxyl product. The carboxyl product is formed primarily at elevated temperatures through the formation of a phosphonium ylide intermediate. The formation of the carboxyl product leads to the formation of the neutral azole, which can lead to an irreversible process if the neutral azole is sufficiently volatile. To understand how the ILs would behave in a CO₂ capture process operated at elevated temperatures, it was necessary to quantify the two reaction products. CO₂ was reacted with seven different AHA ILs to determine the equilibrium amounts of carbamate and carboxyl, the equilibrium constants for both reactions, and the rate of CO₂ absorption by each reaction path. The reactions were tracked and quantified in situ by using ATR-FTIR spectroscopy, while NMR spectroscopy was used after equilibrium was reached to determine the extent of each reaction at multiple temperatures and pressures. It was found that both the basicity and molecular size of the anion play key roles in the formation of the phosphonium ylide. In the extreme case of [P₆₆₆₁₄][4-Triaz] only half of the reacted product was the desired carbamate at 60 °C. Although there is a significant amount of the carboxyl product formed, the carbamate is kinetically favored.

Understanding the Photoexcitation of Room Temperature Ionic Liquids

Photoexcitation of (neat) room temperature ionic liquids (RTILs) leads to the observation of transient species that are reminiscent of the composition of the RTILs themselves. In this minireview, we summarize state-of-the-art in the understanding of the underlying elementary processes. By varying the anion or cation, one aim is to generally predict radiation-induced chemistry and physics of RTILs. One major task is to address the fate of excess electrons (and holes) after photoexcitation, which implies an overview of various formation mechanisms considering structural and dynamical aspects. Therefore, transient studies on time scales from femtoseconds to microseconds can greatly help to elucidate the most relevant steps after photoexcitation. Sometimes, radiation may eventually result in destruction of the RTILs making photostability another important issue to be discussed. Finally, characteristic heterogeneities can be associated with specific physicochemical properties. Influencing these properties by adding conventional solvents, like water, can open a wide field of application, which is briefly summarized.

Mechanical heterogeneity in ionic liquids

Molecular dynamics (MD) simulations of five ionic liquids based on 1-alkyl-3-methylimidazolium cations, [C _n C₁im]⁺, have been performed in order to calculate high-frequency elastic moduli and to evaluate heterogeneity of local elastic moduli. The MD simulations of [C _n C₁im][NO₃], n = 2, 4, 6, and 8, assessed the effect of domain segregation when the alkyl chain length increases, and [C₈C₁im][PF₆] assessed the effect of strength of anion-cation interaction. Dispersion curves of excitation energies of longitudinal and transverse acoustic, LA and TA, modes were obtained from time correlation functions of mass currents at different wavevectors. High-frequency sound velocity of LA modes depends on the alkyl chain length, but sound velocity for TA modes does not. High-frequency bulk and shear moduli, K _∞ and G _∞ , depend on the alkyl chain length because of a density effect. Both K _∞ and G _∞ are strongly dependent on the anion. The calculation of local bulk and shear moduli was accomplished by performing bulk and shear deformations of the systems cooled to 0 K. The simulations showed a clear connection between structural and elastic modulus heterogeneities. The development of nano-heterogeneous structure with increasing length of the alkyl chain in [C _n C₁im][NO₃] implies lower values for local bulk and shear moduli in the non-polar domains. The mean value and the standard deviations of distributions of local elastic moduli decrease when [NO₃]^- is replaced by the less coordinating [PF₆]^- anion.

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, ced_IP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, ced_C+A, where the ionic vapour constituents are isolated ions. A ced_IP dataset is presented for 64 ILs. For the first time an experimental ced_C+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. ced_C+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between ced_IP and the inverse of the molecular volume. A good linear correlation is found between IL ced_IP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to ced_IP. These findings show that ced_IP is very important for understanding IL intermolecular interactions, in spite of ced_IP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.