Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.2 Ionic liquids 1461
be used. This lack of predictive power and the overall general lack of physical property data
in the literature will inevitably slow the transfer of ionic liquid technology into industrial
processes.
21.2.2 FUNDAMENTAL PRINCIPLES OF THE FORMATION OF ROOM
TEMPERATURE IONIC LIQUIDS
A number of detailed reviews and articles have been published recently on the theory and
development behind ionic liquid technology. 15-25 However in order to develop a greater understanding
and appreciation of these solvents, we will summarize the main points here.
Therefore the purpose of this section is to give an introduction to the nature and properties
of room-temperature ionic liquids, with particular emphasis being placed upon their potential
as solvents for industrially relevant catalytic reactions, and (more generally) for clean
technology.
21.2.2.1 Development of ionic liquids
To begin with, the melting point of a salt is related to its lattice energy. In fact, if one was to
plot the lattice energy of a series of salts, for instance the Group 1 halides, against the melting
points in Kelvin, then one can see that there is reasonable linearity between the melting
point and the lattice energy (see Figure 21.2.1).
Although not particularly accurate, this simple approach will give an estimated melting
point of the Group 1 salts if the lattice energy is known. The deviations from this treatment
are frequent and are usually due to other forms of bonding within the structure, as shall
be explained later. The first theoretical treatment of lattice energy began with Born and
Landé, and was then further developed by Kapustinskii 28 into the what is know as the
“Kapustinskii equation” (eqn. [21.2.1]):
+ −
287. 2vZ
Z ⎛ 0. 345 ⎞
U = ⎜ −
r ⎜
1 ⎟
⎝ r ⎟
0 0 ⎠
[21.2.1]
where:
U lattice energy
v number of ions per molecule
r0 sum of ionic radii
Z + ,Z -
charge of the ionic species
Therefore by increasing the value of r0, i.e., by using larger anionic and cationic components
in the salt, it is possible to lower this energy and therefore reduce the melting point.
This effect is shown Table 21.2.1.
Table 21.2.1. Melting points 27 of various inorganic salts, melting points given in ºC
Anion Lithium Sodium Potassium Rubidium Caesium
Fluoride 842 988 846 775 682
Chloride 614 801 776 718 645
Bromide 550 747 734 693 636
Iodide 450 661 686 647 626
From this one can clearly see that as we increase both the size of the anion and the cation,
the melting point decreases. From the Kapustinskii equation one must also note that by