Chemistry Reference
In-Depth Information
stable thermally, mechanically and electrochemically. Furthermore, it can impart very
appealing solvent properties and immiscibility with water or organic solvents which can
be used to create biphasic systems. Some key-properties of ionic liquids are
summarized in Table 1.
[8, 9]
Property
typical values
Melting point
< 100 °C
Liquidus range
> 100-200 °C
Viscosity
< 100 - 150 cP
Thermal stability
High
Vapor pressure
Negligible
Dielectric constant
< 20 - 40
Specific conductivity
< 10 - 15 mS cm
-1
Electrochemical window
Up to 6 V
Table 1
Key properties of modern organocation-based ionic liquids.
[8, 9]
A general synthetic approach for obtaining ionic liquids is the pairing of sterically
demanding cations with a range of organic or inorganic anions. Most modern ionic
liquids are formed when bulky organic cations such as alkylammonium, alkylpyridinium
or imidazolium cations are combined with a range of weakly interacting organic or
inorganic anions such as Cl
-
, BF
4
-
or PF
6
-
or CF
3
SO
3
2-
. Due to their unrivalled properties
(high thermal/chemical stability in combination with low vapour pressure and good
solvent properties) ILs have become one of the fastest growing fields of research in
recent years. The kind of ions used in a ionic liquid essentially influence its viscosity,
solubility behavior, melting point as well as its thermal and electrical stability. For this
reason ILs made a name for themselves as
"designer solvents".
[10]
Some of the typical
ions used are shown in Figure 3 on the next page.
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