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 (Delta(fus)G) 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 (Delta(latt)G)
were estimated using a combination of Volume Based Thermodynamics
(VBT) and quantum chemical calculations. Free energies of solvation
(Delta(solv)G) 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 10(5) Pa) Delta(fus)G
degrees 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 (s(est)) of 8 degrees C. A
similar comparison for dielectric constant predictions gave s(est)
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.
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