Why Are Ionic Liquids Liquid? A Simple Explanation Based on Lattice and Solvation Energies

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Abstract
We have developed a simple and quantitative explanation for the relatively low melting temperatures of ionic liquids (ILs). The basic concept was to assess the Gibbs free energy of fusion (ΔfusG) for the process IL(s) → IL(l), which relates to the melting point of the IL. This was done using a suitable Born−Fajans−Haber cycle that was closed by the lattice (i.e., IL(s) → IL(g)) Gibbs energy and the solvation (i.e., IL(g) → IL(l)) Gibbs energies of the constituent ions in the molten salt. As part of this project we synthesized and determined accurate melting points (by DSC) and dielectric constants (by dielectric spectroscopy) for 14 ionic liquids based on four common anions and nine common cations. Lattice free energies (ΔlattG) were estimated using a combination of Volume Based Thermodynamics (VBT) and quantum chemical calculations. Free energies of solvation (ΔsolvG) of each ion in the bulk molten salt were calculated using the COSMO solvation model and the experimental dielectric constants. Under standard ambient conditions (298.15 K and 105 Pa) ΔfusG° was found to be negative for all the ILs studied, as expected for liquid samples. Thus, these ILs are liquid under standard ambient conditions because the liquid state is thermodynamically favorable, due to the large size and conformational flexibility of the ions involved, which leads to small lattice enthalpies and large entropy changes that favor melting. This model can be used to predict the melting temperatures and dielectric constants of ILs with good accuracy. A comparison of the predicted vs experimental melting points for nine of the ILs (excluding those where no melting transition was observed and two outliers that were not well described by the model) gave a standard error of the estimate (sest) of 8 °C. A similar comparison for dielectric constant predictions gave sest as 2.5 units. Thus, from very little experimental and computational data it is possible to predict fundamental properties such as melting points and dielectric constants of ionic liquids.