Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.2 Ionic liquids 1465
Table 21.2.4 Selected melting points of
the Group 1 chloride mixtures 48
System Mole ratio m.p., °C
LiCl-LiF 70:30 496
LiCl-LiI 35:65 368
LiCl-NaCl 25:75 551
LiCl-CsCl 60:40 355
NaCl-KCl 50:50 658
CsCl-KCl 35:65 610
Table 21.2.5. The melting points of
selected tetrachloroaluminate(III) salts 48
System Mole ratio m.p., °C
LiCl-AlCl3 35:65 80
LiCl-AlCl3 50:50 132
NaCl-AlCl3 39:61 108
NaCl-AlCl3 50:50 151
RbCl-AlCl3 30:70 148
RbCl-AlCl3 50:50 336
[Cn-mim][BF4] ionic liquids. 45-47 This procedure has produced a lower melting point salt for
example the [C4-mim][PF6] has a melting point of 5 o C whereas the [C4-mim][Cl] has a melting
point of 80 o C. These lower melting point liquids with the shorter alkyl chains lead to a
much more fluid and easily managed liquid.
An interesting feature of these phase diagrams is the appearance of liquid crystalline
phases with the longer alkyl chains, and this is confirmed when their optical textures are examined.
35 The implication of the existence of these stable phases has still to be explored in
terms of stereochemical control of reactions.
From these figures it can be seen that those salts with short alkyl chains (n=2-10) are
isotropic ionic liquids at room temperature and exhibit a wide liquid range, whereas the longer
chain analogues are low melting mesomorphic crystalline solids which display an enantiotropic
smectic A mesophase. The thermal range of the mesophase increases with
increasing chain length and in the case of the longest chain salt prepared, [C18-mim][BF4], the mesophase range is ca. 150 o C.
21.2.2.2 Binary ionic liquid systems
In Table 21.2.1 it was shown that the melting points of the Group 1 salts are significantly
above room temperature, and far too high to form a generic medium for reactive chemistry.
However by both increasing the size of the cation and the anion it has become possible to
produce salts that are liquid at room temperature. It is well known that mixing together different
salts deforms the crystal structure, leading to a lower lattice energy, and hence a
lower melting point. At certain concentrations, referred to as eutectic points, the melting
point has reached its minimum. This effect on melting point, obtained by combining the
Group 1 chlorides into various mixtures can be seen in Table 21.2.4.
The melting points of the simple tetrachloroaluminate(III) salts of both the sodium and
[C2-mim] cations have been shown to be significantly lower than their respective chloride
salts, indeed [C2-mim][AlCl4] was shown to be a liquid at room temperature. Such salts are
produced by combining equimolar quantities of either NaCl or [C2-mim]Cl with AlCl3. However, as can be seen from Table 21.2.5, the 50:50 mole ratio does not usually correspond
to the lowest melting point.
For the inorganic salts we now have melting points which are in the maximum range
of some of the high-boiling organic solvents (e.g., 1,4-dichlorobenzene, b.p. 174 o C). Hence
given that the 50:50 mole ratio does not correspond to the lowest melting point of the inorganic
salts, one would expect the same to be true for the organic cations. Figures 21.2.8 and