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

We have developed a simple and quant. explanation for the relatively low melting temps. of ionic liqs. (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 m.p. 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 detd. accurate m.ps. (by DSC) and dielec. consts. (by dielec. spectroscopy) for 14 ionic liqs. based on four common anions and nine common cations. Lattice free energies (ΔlattG) were estd. using a combination of Vol. Based Thermodn. (VBT) and quantum chem. calcns. Free energies of solvation (ΔsolvG) of each ion in the bulk molten salt were calcd. using the COSMO solvation model and the exptl. dielec. consts. Under std. ambient conditions (298.15 K and 105 Pa) ΔfusG° was found to be neg. for all the ILs studied, as expected for liq. samples. Thus, these ILs are liq. under std. ambient conditions because the liq. 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 temps. and dielec. consts. of ILs with good accuracy. A comparison of the predicted vs exptl. m.ps. for nine of the ILs (excluding those where no melting transition was obsd. and two outliers that were not well described by the model) gave a std. error of the est. (sest) of 8 °C. A similar comparison for dielec. const. predictions gave sest as 2.5 units. Thus, from very little exptl. and computational data it is possible to predict fundamental properties such as m.ps. and dielec. consts. of ionic liqs.