Chemistry Reference
In-Depth Information
13.1
Introduction: A Survey on IL Challenges
In 1914 the first ionic liquid (IL) was mentioned as the ethyl ammonium nitrate
(Walden
1914
). Research in this area went diffident, since in 1948 the physical and
chemical properties of pyridinium aluminium tetrachloride have been investigated,
and the first review in more detail about IL based on tetraalkylammonium cation with
chloroaluminate anions appeared only in 1983 (Hussey
1983
). In 1990's came the
first reports of ILs based on imidazolium cation, stable in air and water, which predict
some possible uses of these salts (Wilkes and Zaworotko
1992
). Thus was sparked the
interest of researchers, it is actually the start for future intensive research of ILs, with
more surprising results, ILs offering unexpected opportunities in numerous fields
(Zhang et al.
2009
; Mallakpour and Rafiee
2011a
,
b
).
Ionic liquids are salts, peculiar organic molten salts, stable in air and water,
completely composed of ions, and generally exhibit melting temperatures below
100
◦
C (Stark and Seddon
2007
). Their remarkable characteristics such as: melt-
ing point, their preservation state as liquids within a broad temperature as large as
300
◦
C, dielectric constant, viscosity, density, polarity, water-miscibility, they are
able to dissolve a wide range of organic, inorganic, organometallic compounds and
polymers, low toxicity, negligible vapor pressure, non-flammable, non-explosive,
non-coordinating, non-solvating, not very corrosive, high ion conductivity, the elec-
trochemical window, excellent thermal and chemical stability, ILsS have become a
very interesting topic to research, with applications in many branches of chemistry
and chemical technology and many other areas. (Wei and Ivaska
2008
; Zhang et al.
2009
; Mallakpour and Rafiee
2011a
,
b
).
Usually ILs consist of a organic cation associated with a (poly)atomic anion that
may be either organic or inorganic. Because the symmetrical species tend to pack
more effectively in the solid state, and tend to form salts with higher melting points,
the cations are ordinarily designed with a reduced symmetry, limiting their packing
in a crystal and giving a low melting point (Zhang et al.
2009
; Coleman and Gath-
ergood
2010
). The structure of ILs which is modular nature, and afford structural
modifications that can be made either to the anionic or the cationic component thus
may be obtained a wide diversity of possible ILs with different physicochemical
properties (Zhang et al.
2009
; Coleman and Gathergood
2010
). By modifying the
structural factors of cations and anions: size, symmetry, charge density, chain length,
functional groups, etc., physicochemical properties are fine-tuned (Zhang et al.
2009
;
Coleman and Gathergood
2010
; Pham et al.
2010
). According to the choice of cation
are determine the principal physical properties (e.g., melting point, viscosity, den-
sity, solubility, hydrofobicity, etc.), whilst the choice of the anion affects more the
chemistry (Mallakpour and Rafiee
2011a
,
b
). Special ILs can be designed by chang-
ing cations and anions to meet the specific physical/chemical properties required for
each specific application, in this way can be made task-specific ILs (TSIL) for a
certain application (Wei and Ivaska
2008
; Pham et al.
2010
; Carrera et al.
2010
).
Generally ILs are classified by type cation into: ILs containing aromatic head
groups cation and ILs containing acyclic head groups cation (Petkovic et al.
2011
)
or for being more specific in ILs based on the: imidazolium (IM), pyridinium (Py),
