Handbook of Solvents - George Wypych - ChemTech - Ventech!

1464 D.W. Rooney, K.R. Seddon
Figure 21.2.4. Relationship between the size of similar
anions containing halides and the melting point of
their [C 2-mim] salts.
Figure 21.2.6. Melting point phase diagram for
[C n-mim][PF 6] ionic liquids as a function of alkyl
chain length n showing the melting transitions from
crystalline (closed square) and glassy (open square)
materials and the clearing transition (circle) of the liquid
crystalline (LC) terms.
Figure 21.2.5. Phase diagram for the [C n-mim]Cl ionic
liquids. 44
Figure 21.2.7. Melting point phase diagram for
[C n-mim][BF 4] ionic liquids as a function of alkyl chain
length n showing the melting transitions from crystalline
(closed square) and glassy (open square) materials
and the clearing transition (circle) of the liquid crystalline
terms.
show that the melting point for the [FeCl 4] - ionic liquid does appear to follow the given
trend.
In order to expand the number of potential ionic liquids the possibility also exists to increase
the length of the alkyl group thereby further decreasing the lattice energy. For the
[C n-mim] chlorides the result is a range of salts all of which have low melting points (some
of which are below room temperature) as shown in Figure 21.2.5, though often at the cost of
increased viscosity.
One can observe that there is a maximum chain length allowed before other forms of
bonding begin to dominate and the melting point increases. The melting points indicated below
the 0 o C line are in fact glass transition temperatures rather than true melting points. As
stated above, one point to note is that the viscosity of these room temperature “chloride” liquids
is very high and they would therefore have to be used at relatively high temperatures if
required in a process.
Figures 21.2.6 and 21.2.7 illustrate the significant variation in melting point which can
be induced by simply changing the anion from [C n-mim][Cl] to the [C n-mim][PF 6] and