Quantifying intermolecular interactions of ionic liquids using cohesive energy densities.

Kevin R J Lovelock

doi:10.1098/rsos.171223

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.

Highlights

This article has been edited by the Royal Society of Chemistry, including the commissioning, peer review process and editorial aspects up to the point of acceptance
These findings show that cedIP is very important for understanding ionic liquids (ILs) intermolecular interactions, in spite of cedIP not being a measure of the total intermolecular interactions of an IL
Results for the cohesive energy density for molecular liquids cedML,298 values range from 195 J cm−3 for squalane, through 385 J cm−3 for acetone, 679 J cm−3 for ethanol, 1055 J cm−3 for ethanolamine and up to 2293 J cm−3 for water [32]

Summary

Introduction

This article has been edited by the Royal Society of Chemistry, including the commissioning, peer review process and editorial aspects up to the point of acceptance. The strength of intermolecular interactions have historically been used to understand and predict a range of properties for molecular liquids, including surface tension [11], wettability [12], the ability of a solvent to promote solute self-assembly [13,14], solubility [15] and viscosity [16,17,18,19]. All of these properties are very important for potential IL applications. Establishing links between these and other properties will give significant insight into the properties that underpin a wide range of applications

Methods

Results

Conclusion