Chemistry Reference
In-Depth Information
2007
,
2010
) and also was showed that pyridinium salts exhibit rather low thermal
decomposition temperatures (Scammells et al.
2005
; Patrascu et al.
2004
).
Densities
of ILs is strongly depend on the temperature, water content of ILs,
and the structures of cations and anions. In general, the densities of ILs decrease
with the increase in alkyl chain length and the increase of cation size (Zhang et al.
2009
; Papaiconomou et al.
2007
,
2010
). It was show that because of the absence of
trifluoromethyl groups on the anion, pyridinium-based ILs containing dicyanamide
or thiocyanate anions exhibit low densities (
1 g/cm
3
) and ILs with [NTf
2
] anions
exhibit the highest densities (Papaiconomou et al.
2007
,
2010
).
The refractive indices
(Papaiconomou et al.
2007
,
2010
) for pyridinium-based ILs
are range from 1.4 to 1.6, are higher than those for corresponding imidazolium-based
ILs. The influence of the cation is not very clear, anions have the most important
influence on the refractive index: the refractive index increase with the decrease of the
anion. For the same cation, refractive indices increase in the following anion order:
n(NTf
2
)
<
n(TfO)
<
n(N(CN)
2
)
<
n(SCN)
<
n(I) (Papaiconomou et al.
2007
,
2010
).
The
viscosities
are very sensitive to impurities, water content and temperatures:
the water and organic solvents will decrease the viscosity of ILs, while the presence
of chloride increases it (Zhang et al.
2009
; Wei and Ivaska
2008
; Seddon et al.
2000
).
Presence of cosolvents in ILs reduces the viscosity, with the effect on being stronger
with the higher the dielectric constant of the cosolvent is (Seddon et al.
2000
). The
viscosity at room temperature of ILs are in range from 10 to 500 cP, ILs being more
viscous than molecular solvents. The viscosities of imidazolium ILs have the same
trends to those of the pyridinium IL: increase with an increase in cation alkyl chain
length and also with the number of methyl that are added onto the pyridine ring (Zhang
et al.
2009
; Papaiconomou et al.
2007
,
2010
). Presence of ester or hydroxyl groups
in ILs based on the pyridinium or ammonium cations increases the viscosity, ester
functions having a bigger effect than the hydroxyl functionalities (Deng et al.
2011
).
Commonly lattice energy and ion solvation of salts determine their
water solu-
bility
(Ranke et al.
2009
). For ILs the lattice energy is irrelevant and differences
in hydrophobicity account more for differences in their water solubility (Ranke
et al.
2009
). According to their
solubility in water
ILs can generally be divided
into
water-immiscible (hydrophobic)
and
water-miscible (hydrophilic) ILs
(Wei and
Ivaska
2008
; Ranke et al.
2009
).
Water-immiscible ILs
form non-polarizable inter-
faces in contact with water and they can be easily prepared in aqueous solution. They
can also be obtained in very dry state and they are especially suitable for applications
in electrochemical systems (Wei and Ivaska
2008
; Ranke et al.
2009
).
Water-miscible
(hydrophilic) ILs
have been considered unstable in aqueous solutions but recently
was found that they also can form films with active electrochemical properties on
glassy carbon electrode (Wei and Ivaska
2008
; Ranke et al.
2009
).
The miscibility of ILs in water is strongly dependent on the anions (Wei and Ivaska
2008
; Seddon et al.
2000
; Anthony et al.
2001
). Anions like Cl
−
,Br
−
,I
−
,NO
3
,
CH
3
COO
−
and CF
3
COO
−
are making the ILs miscible with water, while anions
such as PF
6
and Tf
2
N
−
are making them immiscible with water. For ILs contain-
ing anions as BF
4
and CF
3
SO
3
(TfO
−
) the miscibility with water depend on the
structure of the cations (in general are miscible with water) (Wei and Ivaska
2008
;
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