108
M. I. Qadir et al.
this IL was not inert to many organic solvents, very sensitive to moisture and could
easily be degraded to generate other side products including HCl, Al(OH) 3 and
Al 2 O 3 . To circumvent this problem, our group reported the very first air-stable
imidazolium-based ILs containing tetrafluoroborate (BF 4
− ) and hexafluorophosphate (PF 6
− ) anions [7, 8]. With the preparation of these air-stable and easily handled
green solvents, interest in them showed an exponential growth.
Besides imidazolium ILs, phosphonium, pyrrolidinium, pyridinium and aminebased ILs have been prepared and studied. Unlike water and other traditional
solvents, imidazolium-based ILs display high self-organisation on the nanomolecular
scale. 1,3-Dialkylimidazolium ILs, which form one of the most investigated classes,
are better described as hydrogen-bonded polymeric supramolecules of the type
{[(DAI) x (X) x−n )]
n+ [(DAI) x−n (X) x )]
n− } n where DAI is the 1,3-dialkylimidazolium
cation and X the anion [12]. This structural pattern is a general trend for the solid
phase and is maintained to a great extent in the liquid phase, and even in the gas
phase, while upon mixing with other molecules they should be better regarded as
nano-structured materials with polar and nonpolar regions rather than homogeneous
solvents.
ILs are composed of ions (cations and anions) and are generally asymmetric
and flexible with delocalised electrostatic charges. These phenomena result in
more complex Coulombic interactions (mutual electrostatic attraction or repulsion
of charged particles), van der Waals interactions, polarisation and π–π, dipole–
dipole, hydrogen-bonding, and solvophobic interactions. ILs in the bulk state often
exhibit richer well-defined nanostructures with segregated regions of polar and
nonpolar domains in the bulk phase [9]; for example, long alkyl side chain 1,3dialkylimidazolium ILs, in which the alkyl chains can segregate to form nonpolar
domains, while other parts of the IL form polar domains (Fig. 4.1). As the length of
the alkyl chain increases, the nonpolar domains increase in size and become more
connected, leading to a microphase separation, such as in liquid crystal formation.
Anisotropic-like liquids are observed in this case.
4.2 Stabilisation and Characterisation of NPs in ILs
Highly dispersed soluble NPs are desirable for catalysis and solution-processible
optoelectronics. But, NPs are kinetically unstable with respect to agglomeration to
the bulk metal. Therefore, they require stabilisation, which can be achieved by either
surface-ligating anions or other ligands. In this regard, different types of stabilising
agents, such as water-soluble polymers, quaternary ammonium salts, surfactants and
polyoxoanions, have been applied to provide electronic and steric protection [13].
These candidates effectively decrease the efficiency of the soluble NPs through their
strong binding at the NP surface. In this way, ILs, especially imidazolium-based
ILs, have emerged as versatile ionic media that stabilise the formed NPs through
Coulombic forces, van der Waals interactions, polarisation and π–π, dipole–dipole,
hydrogen-bonding and solvophobic interactions. It is believed that the ILs stabilise
M. I. Qadir et al.
this IL was not inert to many organic solvents, very sensitive to moisture and could
easily be degraded to generate other side products including HCl, Al(OH) 3 and
Al 2 O 3 . To circumvent this problem, our group reported the very first air-stable
imidazolium-based ILs containing tetrafluoroborate (BF 4
− ) and hexafluorophosphate (PF 6
− ) anions [7, 8]. With the preparation of these air-stable and easily handled
green solvents, interest in them showed an exponential growth.
Besides imidazolium ILs, phosphonium, pyrrolidinium, pyridinium and aminebased ILs have been prepared and studied. Unlike water and other traditional
solvents, imidazolium-based ILs display high self-organisation on the nanomolecular
scale. 1,3-Dialkylimidazolium ILs, which form one of the most investigated classes,
are better described as hydrogen-bonded polymeric supramolecules of the type
{[(DAI) x (X) x−n )]
n+ [(DAI) x−n (X) x )]
n− } n where DAI is the 1,3-dialkylimidazolium
cation and X the anion [12]. This structural pattern is a general trend for the solid
phase and is maintained to a great extent in the liquid phase, and even in the gas
phase, while upon mixing with other molecules they should be better regarded as
nano-structured materials with polar and nonpolar regions rather than homogeneous
solvents.
ILs are composed of ions (cations and anions) and are generally asymmetric
and flexible with delocalised electrostatic charges. These phenomena result in
more complex Coulombic interactions (mutual electrostatic attraction or repulsion
of charged particles), van der Waals interactions, polarisation and π–π, dipole–
dipole, hydrogen-bonding, and solvophobic interactions. ILs in the bulk state often
exhibit richer well-defined nanostructures with segregated regions of polar and
nonpolar domains in the bulk phase [9]; for example, long alkyl side chain 1,3dialkylimidazolium ILs, in which the alkyl chains can segregate to form nonpolar
domains, while other parts of the IL form polar domains (Fig. 4.1). As the length of
the alkyl chain increases, the nonpolar domains increase in size and become more
connected, leading to a microphase separation, such as in liquid crystal formation.
Anisotropic-like liquids are observed in this case.
4.2 Stabilisation and Characterisation of NPs in ILs
Highly dispersed soluble NPs are desirable for catalysis and solution-processible
optoelectronics. But, NPs are kinetically unstable with respect to agglomeration to
the bulk metal. Therefore, they require stabilisation, which can be achieved by either
surface-ligating anions or other ligands. In this regard, different types of stabilising
agents, such as water-soluble polymers, quaternary ammonium salts, surfactants and
polyoxoanions, have been applied to provide electronic and steric protection [13].
These candidates effectively decrease the efficiency of the soluble NPs through their
strong binding at the NP surface. In this way, ILs, especially imidazolium-based
ILs, have emerged as versatile ionic media that stabilise the formed NPs through
Coulombic forces, van der Waals interactions, polarisation and π–π, dipole–dipole,
hydrogen-bonding and solvophobic interactions. It is believed that the ILs stabilise