Chemistry Reference
In-Depth Information
Impurities that can be found in ILs are: water, halides, unreacted organic starting
materials and compounds from the incomplete metathesis reaction, this are affecting
the physicochemical properties of ILs (e.g.: density, viscosity, their decomposition,
etc.) (Zhang et al.
2009
; Scammells et al.
2005
). In some cases they do not have
a very big impact, such as the Heck reaction that is unaffected by the presence
of impurities. But in other cases or in the case of certain very sensitive chemical
applications (catalysis, electrochemical and spectroscopic application), the ILs must
be of high purity (Scammells et al.
2005
; Zhang et al.
2009
).
As mentioned above the
physical properties
of ILs can be tailored by a variation
in the cationic and the anionic precursor.
Usually when an IL is heated from the solid state cannot pass directly in liquid
state, is going through several phase change points such as glass transition point
(Tg), melting point (Tm) and clearing point (Tc) (Zhang et al.
2009
). ILs can exhibit
the supercooled phenomena, and may form supercooled liquids when the freezing
point (Tf) is not consistent with melting point (Tm) and the gap between them is as
wide as 40-90 K (Zhang et al.
2009
).
Foralow
melting points
, the cations are typically unsymmetric (Zhang et al.
2009
; Coleman and Gathergood
2010
) and also we should take account that ILs
with shorter alkyl chains are ruled by electrostatic forces and ILs with longer
alkyl chains are ruled by the short-range van der Waals interactions (Zhang et al.
2009
). Papaiconomou et al. (
2007
,
2010
) find that in many cases the melting point
is higher for the imidazolium or pyridinium cations substituted with methyl sub-
stituents than those ionic liquids containing no alkyl substituents. Also they find
that some pyridinium-based ILs containing iodine anions are solid at room temper-
ature (e.g. [Py8-3Me-5Me][I]) and for ILs containing [TfO]
−
anions, the addition
of an extra methyl group on pyridine ring increases melting temperature; ILs con-
taining butyl chains and [NTf
2
]
−
anions exhibited Tg and no Tm, whatever the
alkyl chain or cation ring was used (Papaiconomou et al.
2007
,
2010
) and for the
same anion and pyridinium ring, the ionic liquids containing butyl chains exhibit
lower Tg values than those containing octyl chains (Papaiconomou et al.
2007
,
2010
). The melting points of ILs generally increase with the decreasing of anion
size Cl
−
>
Br
−
>
[PF
6
]
−
>
[BF
4
]
−
>
[TfO]
−
>
[TFSI]
−
(Zhang et al.
2009
).
ILs usually have good
thermal stability
which is a little bit influenced by the
organic nature of the cations/anions that restricts their upper stability temperature,
defined by the decomposition point (Td) (Zhang et al.
2009
). The thermal stability
of ILs is dominated by the charge density, acidic proton, and expansionary force of
cations and anions. The influence of the pyridinic cation on thermal stability is not
so clear, and it has been observed that imidazolium ILs are more stable than the cor-
responding ammonium and pyridinium ILs (Zhang et al.
2009
; Papaiconomou et al.
2007
,
2010
). Degradation of ionic liquids is mostly influenced by the nature of the an-
ion, its nucleophilicity (Scammells et al.
2005
; Papaiconomou et al.
2007
,
2010
). ILs
containing weakly coordinating anions are mostly stable to high-temperature decom-
position (Zhang et al.
2009
). For any pyridine cation studied by Papaiconomou et al.
(
2007
), the decomposition temperatures of the ionic liquids are in the following an-
ion order: [I]
<
[SCN]
<
[N(CN)
2
]
<
[NTf
2
]
≤
[TfO]
<
[BF
4
] (Papaiconomou et al.
