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.