Although the information about physical, thermodynamic,
kinetic or engineering data had been extended continuously, only little
data with regard to the toxicity
and ecotoxicity of ionic
been available until now.
The “green character” of ionic liquids
has usually been justified with
their negligible vapour pressure, but even if ionic liquids will not
evaporate and will not contribute to air pollution most of them are
water soluble and might enter the environment by this path (e.g.
accidental spills, effluents). Until now we do not know what the
consequences are in these cases.
In general there are different ways to approach the
question of toxicity.
- On the one side properties as (eco)toxicity,
degradability of ionic liquids could be discussed considering the
chemical structures and using the knowledge on
Structure-Activity-Relationships (T-SAR, QSAR) or
- On the other side (eco)toxicological
test methods will be always necessary to corroborate these working
hypothesis and to get information about the potential effects of new
compounds in complex systems which can be predicted only with great
Continuous interaction of both approaches result in a
mutual optimisation process.
- Referring to their chemical structure
Wasserscheid et al. (2002)
pointed out that in the case of ionic liquids
consisting of halogen
containing anions with poor stability in water, toxic and corrosive
species such as HF or HCl might be liberated. Therefore they suggest
the use of halogen-free and relatively hydrolysis-stable anions such as
- Gathergood et al. (2004) synthesised ionic liquids
side chains. These modifications should allow enzymatic hydrolysis.
Using the OECD - “Closed Bottle Test” an increase in biodegradability
could be shown.
- Stock et al. (2004) presented initial results on
molecular toxicity of ionic liquids with the acetylcholinesterase
inhibition assay using the purified enzyme from electric eel
(Electrophorus electricus). The results indicate that longer alkyl
chain length (position R2) of imidazolium ionic liquids resulted in a
stronger inhibition of the enzyme. Furthermore the strongest inhibition
was obtained with ionic liquids containing positively charged nitrogen
(pyridinium and imidazolium). The ionic liquids based on imidazolium
were slightly less inhibitory and the phosphonium compounds showed the
smallest inhibition potential.
- Other studies showed that in certain cases ionic
might even be
used as reaction media for enzymatic reactions or other
biotransformations. For example, lipases are active in ionic liquids
containing BF4 or PF6 anions and higher yields in
biocatalysis could be realised due to better enzyme stability,
substrate and/or product selectivity and suppression of side reactions.
(Schöfer et al. 2001, Kragl et al 2002).
- The antimicrobial activity of a series of
3-alkoxymethyl-1-methylimidazolium salts of Cl-, BF4-
and PF6- on
different bacteria (Cocci, rods) and fungi had been tested by Pernak et
al. (2002). The results showed that elongation of the alkyl chain
length in the alkoxymethyl substituent increased the antimicrobial
activity. Further the antimicrobial activity had not been affected by
the type of anion. Ranke et al. (2004) exposed the marine bacterium
Vibrio fischeri to different concentrations of alkylimidazolium salts
of BF4-, PF6-, o Cl-
and Br-. The inhibition of luminescence as well
increased with increasing n-alkyl chain length in R2-position but no
systematic influence of the anion could be determined.
- Higher loss of cell viability with increasing chain
alkyimidazolium ionic liquids had been also observed using mammalian
cells (IPC 81 leukemia cells, C6 glioma cells) (Ranke et al. 2004).
- Even experiments with the soil nematode
that lethality of these organisms increased in the presence of
alkylimidazolium salts of Cl- with increasing alkyl chain
(Swatlowski et al. 2004).
toxicity studies are reported for
LD50 = 1400 mg/kg and 1370 mg/kg, for male and female rats,
respectively, are reported. 4
- At the University of Notre Dame (IN, USA) research in
respect to ecological impacts on aquatic ecosystems has already
commenced (see below).
impact on aquatic
The impact ionic liquids have on aquatic ecosystems is
particularly important, given their non-volatile nature and mild to
high solubility in water of some of them.
A research performed in collaboration with Prof. Joan
Brennecke at the University of Notre Dame (IN, USA) in 2004
provides the LC50 levels for two imidazolium-based ionic liquids with
Daphnia are a common fresh water crustaceans. They are
filter feeders, and are at the base of the aquatic foodchain. Their
response to ionic liquids is therefore crucial for assessing how these
new solvents will impact an environmental ecosystem.
Image of a
water flea (Daphnia magna)
The following LC50
were obtained for 1-n-butyl-3-methylimidazolium cations with PF6
As can be seen, these two ionic liquids are about as
toxic to Daphnia as benzene, and are far more toxic than acetone.
The first results of (eco)toxicological effects of ionic
liquids indicate that due to the structure of the compounds similar
trends might occur in test systems of different biological complexity
(e.g. enzymes, micro organisms, organisms) and with different endpoints
(e.g. lethality, reproduction) although the absolute effect levels
might be distinct.
These corresponding effects might be explained with the
properties of longer alkyl chains and the similarity of imidazolium
compounds to cationic surfactants. Cationic surfactants are known to
increase membrane permeability and therefore cause narcotic effects
with increasing chain length (Cross, 1994).
First results give reason to be optimistic that with
increasing data of (eco-) toxicological properties of selected ionic
liquids it might be possible to reduce the complexity of chemical
structures of ionic liquids which could be synthesised by chemists by
means of theoretical assessment of possible biological activities with
help of T-SAR prior to synthesis.
At least it should be mentioned that for all
(eco)toxicological assays attention has to be paid to the purity of the
ionic liquids used. Otherwise side effects of substances which had been
used in access during the synthesizing or following purification
processes (Swatloski et al. 2004) cannot be excluded. Therefore
different authors (Swatloski et al. 2004, Stepnowski et al. 2003)
attached significant importance to proper analysing techniques.12