Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

Abstract

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, cedIP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, cedC+A, where the ionic vapour constituents are isolated ions. A cedIP dataset is presented for 64 ILs. For the first time an experimental cedC+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. cedC+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 cedIP and the inverse of the molecular volume. A good linear correlation is found between IL cedIP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to cedIP. These findings show that cedIP is very important for understanding IL intermolecular interactions, in spite of cedIP 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.

Thermal stability of dialkylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids: ex situ bulk heating to complement in situ mass spectrometry

“…As far as we are aware, this is the first reported observation of the [BF4]anion in negative mode MS after vaporization of an IL. 33 Overall, these observations confirm that at 500 K under these experimental conditions the vaporization of [C4C1Im][BF4] NIPs is the dominant process.…”

“…Vaporization of the NIPs occurs at much lower temperatures than liquid phase TD. Therefore, it can also be concluded that for 27,33,34,40,60 indicating that Ea,TD for these two ILs are likely to be towards 200 kJ mol -1 .…”

“…Although C8C1(C3N2H2)PF5 was the major product, the [C8C1Im][PF6] condensate contained less TD product and more IL than the [C4C1Im][PF6] condensate. As the condensation temperature was slightly lower for [C8C1Im][PF6] (albeit by 16 K), IL vaporization should be favored over TD; however, longer alkyl chain ILs have higher vapH values, 33 suggesting less IL should be collected in the condensate. Interestingly, the [C8C1Im][PF6] residue contained only 2% C8C1(C3N2H2)PF5, the same proportion as the higher temperature [C4C1Im][PF6] condensate.…”

“…32 A large number of studies have subsequently focused on investigating IL vaporization, most utilizing ILs containing the thermally robust [NTf2]anion. 33 Mass spectrometry (MS) 22,27, and simulations 44,[56][57][58][59] have since identified the vapor phase of ILs to be composed primarily of neutral ion pairs (NIPs), including for both [CnC1Im][BF4] and [CnC1Im][PF6] (n = 2, 4, 8). 34,36,39,40,42 Using temperature programmed desorption (TPD) line-of-sight mass spectrometry (LOSMS) only IL NIP vaporization was observed; no evidence was found for the TD of [CnC1Im][BF4] to CnC1(C3N2H2)BF3 or [CnC1Im][PF6] to CnC1(C3N2H2)PF5 between 450 K to 565 K. 34,36,39,40 Note that other studies where IL NIP vaporization was detected for [CnC1Im][BF4] to or [CnC1Im][PF6] did not report CnC1(C3N2H2)BF3 or CnC1(C3N2H2)PF5 formation.…”

Separation of the main flavonoids and essential oil from seabuckthorn leaves by ultrasonic/microwave-assisted simultaneous distillation extraction

“…The anion is considered to be the significant factor influencing the properties of ILs [ 35 ]. 1-Alkyl-3-methylimidazolium-based ILs are widely used in sample preparation and compounds extraction from plant materials, thus, 1-Butyl-3-methylimidazolium-based ILs with the same concentrations but six different anions (Br − , Cl − , BF4 − , ClO 4 − , HSO 4 − and NO 3 − ) were used in UMASDE to identify the best IL anion [ 43 ]. The results ( figure 4 a ) show that the yields of both EOs and the four flavonoids obtained using [C 4 mim]Br was higher than those obtained using other ILs.…”