Chemistry Reference
In-Depth Information
The
biodegradability of ILs
depends on the structure of the cation—especially on
the length of the alkyl side chain (the longer is alkyl side chain (
>
C6) the better the
biodegradability is) with the exception of octyl side chain which have a disputable
effect on biodegradation (Coleman and Gathergood
2010
; Pham et al.
2010
; Petkovic
et al.
2011
; Abrusci et al.
2011
; Stolte et al.
2008
; Gathergood and Scammells
2002
;
Romero et al.
2008
). But with increasing alkyl side chain length it is also increasing
the toxicity to microorganisms, effect that can be attenuated by introduction of ester
functional groups into the alkyl side chain of imidazolium and pyridinium based ILs
(Coleman and Gathergood
2010
; Pham et al.
2010
; Petkovic et al.
2011
; Abrusci
et al.
2011
; Gathergood et al.
2004
; Garcia et al.
2005
). The biodegradability of
ILs also depends on the structure of anion: organic anions are usually biodegradable
(e.g. [OctSO
4
]
−
, [CH
3
COO]
−
) (Coleman and Gathergood
2010
; Pham et al.
2010
;
Petkovic et al.
2011
; Abrusci et al.
2011
;Yuetal.
2008
), and for inorganic anions, the
biodegradability 18 decreased in the order PF
6
>
BF
4
>
Br
−
>
Cl
−
(Coleman and
Gathergood
2010
; Pham et al.
2010
; Petkovic et al.
2011
; Abrusci et al.
2011
; Ranke
et al.
2007
). The presence of functional oxygenated moieties reduce the lipophilicity
of ILs (including pyridine) leading to a decrease in the risk of bioaccumulation in
environment (Denga et al.
2012
). A warning sign is that in general the most common
ILs shown to be not readily biodegradable (Petkovic et al.
2011
; Abrusci et al.
2011
).
Every ionic liquid is unique and has a distinct individual toxicity and biodegrad-
ability (Coleman and Gathergood
2010
). In general ILs are more toxic than many
traditional solvents to aquatic organisms and an increase hydrophobicity (lipophilic-
ity) and toxicity correspond to an increase in alkyl chain length of the substituted
groups, ILs with longer alkyl chains possess more lipophilic properties, the shorter
the chain length(s) of side chain(s), the lower the cytotoxicity (Zhang et al.
2009
;
Coleman and Gathergood
2010
; Pham et al.
2010
; Petkovic et al.
2011
; Frade and
Afonso
2010
; Ranke et al.
2004
; Jastorff et al.
2005
; Stepnowski et al.
2004
). Jas-
torff and coworkers observed a good correlation between the cations lipophilicity
and the toxicity of the corresponding halide ILs but there are some exceptions of
very lipophilic cations or anions (side chains
>
C10), that exhibited lower toxicity
than that predicted (Stolte et al.
2007a
,
b
). Aquatic toxicity can be the result from
disruption of the membrane by the compounds (Scammells et al.
2005
). This can be
made by a hydrophobic/ionic adsorption at the cell membrane/water interface, since
this hydrophobic molecules tend to accumulate at this interface probably toxicity
will increase with the increase of hydrophobicity (Scammells et al.
2005
).
ILs toxicity seems to be influenced mainly by the characteristics of the cation
which can be explained by intercalation of the lipophilic part of the molecules into the
membrane, whereas the ionic headgroup is at least partially solvatized in the aqueous
solution (Zhang et al.
2009
; Ranke et al.
2004
), also toxicity being dependent on the
incubation time (Cho et al.
2008
).
A theoretical study predicted that expected toxicity should increase in the order
ammonium
<
pyridinium
<
imidazolium
<
triazolium
<
tetrazolium, with increas-
ing the number of nitrogen atoms in an aromatic cation ring (Couling et al.
2006
).
Toxicity of the ILs cations to IPC-81 seems to increase in the order: [Mor]
<
[Pip]
<
[Py]
<
[Pyr]
<
[IM]
<
[Quin]
<
[N
4444
]
<
[P
4444
], and in MCF-7 cells increase in a
