Bernd Jastorff

Biological effects of imidazolium ionic liquids with varying chain lengths in acute Vibrio fischeri and WST-1 cell viability assays

Detailed biological studies of methyl- and some ethylimidazolium ionic liquids in luminescent bacteria as well as in the IPC-81 (leukemia cells) and C6 (glioma cells) rat cell lines are presented. Effective concentrations in these test systems are generally some orders of magnitude lower than effective concentrations [corrected] of the conventional solvents acetone, acetonitrile, methanol, and methyl t-butyl ether. No general influence of the anionic compound in the ionic liquids on toxicity could be found, although they seem to modulate toxicity in some cases. The clear influence of the alkyl chain length on toxicity was quantified by linear regression analysis. Alkyl chain length of the longer alkyl chain was varied from 3 to 10 carbon atoms. Consequences for a design of sustainable alternative solvents are briefly sketched.

Effects of different head groups and functionalised side chains on the aquatic toxicity of ionic liquids

In this study, the influence of different head groups, functionalised side chains and anions of ionic liquids on the marine bacteria Vibrio fischeri, the limnic green algae Scenedesmus vacuolatus and the fresh water plant Lemna minor was investigated. The aim of these experiments is to improve the knowledge base for the molecular design of ionic liquids leading to a reduced (eco)toxicological hazard potential. The analysed set of about 40 ionic liquids confirmed the interdependency between lipophilicity—as derived from gradient HPLC—and (eco)toxicity. The toxicity was clearly reduced for the test organisms (partially by six to seven orders of magnitude) when short functionalised side chains were used instead of non-polar alkyl chains. Furthermore, we could demonstrate strong interactions of hydrophobic ionic liquid cations with two different types of common biological lipidbilayers, indicating that the membrane system of organisms is probably a primary target site of toxic action. These systematic studies are addressed to producers, developers and downstream users of ionic liquids in different fields of application, to facilitate the selection of (eco)toxicologically favourable structural elements and thus to contribute to the design of inherently safer ionic liquids.

How hazardous are ionic liquids? Structure–activity relationships and biological testing as important elements for sustainability evaluationThis work was presented at the Green Solvents for Catalysis Meeting held in Bruchsal, Germany, 13–16th October 2002.

For ionic liquids only few toxicological and/or ecotoxicological data are available until now. A strategy is presented which aims at an environmental risk assessment of chemicals, using a combination of structure–activity relationships (SAR), toxicological and ecotoxicological tests and modelling. The parts “test-kit-concept” and “multidimensional risk analysis” are described in detail by means of selected imidazolium ionic liquids. The iterative process of this strategy offers a tool for sustainable product design.

The influence of anion species on the toxicity of 1-alkyl-3-methylimidazolium ionic liquids observed in an (eco)toxicological test battery

To enlarge the restricted knowledge about the hazard potentials of ionic liquids to man and the environment we have concentrated on analysing systematically the anion effect of six different anion moieties (Cl–, BF4–, (CF3SO2)2N–, (CF3)2N–, octylsulfate and bis(1,2-benzenediolato)borate) and the influence of the side chain length at the cation on (eco)toxicity. For our investigations, we used the flexible (eco)toxicological test battery considering aquatical and terrestrial compartments as well as different trophic levels including enzymes (acetylcholinesterase), mammalian cells (IPC-81), luminescent marine bacteria (Vibrio fischeri), limnic unicellular green algae (Scenedesmus vacuolatus), wheat (Triticum aestivum), cress (Lepidium sativum), duckweed (Lemna minor) and a soil invertebrate (the spring tail Folsomia candida). In general, the side chain effect was found consistently in all used test systems from the molecular up to the organismic level. Such a consistent response of the different test systems could not be confirmed for the tested anion moieties. Furthermore, in most of the investigated test systems the anion effects are not as distinct as the demonstrated side chain length effect. Nevertheless, for (CF3SO2)2N– a clear (eco)toxicological hazard potential is evident. Thus, the strategy to check toxicities within a flexible (eco)toxicological test battery has been proven to be effective for uncovering hazard potentials of ionic liquids.

Anion effects on the cytotoxicity of ionic liquids

Most recent investigations concerning the toxicological and ecotoxicological risk potentials of ionic liquids are predominantly focusing on the cation moieties. In this study we elucidate, whether the anion species commonly used in ionic liquids are exhibiting intrinsic cytotoxic effects and if these effects can be rationalised by thinking in terms of structure–activity relationships (T-SAR). As test system to measure the cell viability as toxicologically relevant endpoint the IPC-81 rat leukemia cell line and the WST-1 assay were employed. Our results show an anion effect in ionic liquids on cytotoxicity for 10 of 27 tested anions. For the remaining 17 anions from our test kit no significant effect was found. With respect to structure–activity relationships, lipophilicity and/or vulnerability to hydrolytic cleavage seem to be the key structural features leading to the observed anion cytotoxicity. We also conclude that the model of concentration addition may be useful to estimate the EC50 values of ionic liquids that have not been tested or even synthesised yet. This can help to design not only task specific but also inherently safer ionic liquids.

Progress in evaluation of risk potential of ionic liquids—basis for an eco-design of sustainable products

Motivated by the prevailing need for a sustainable development and taking the principles of Green Chemistry as a starting point, the present paper describes new and updated findings regarding a sustainable product design for ionic liquids. The focus is on environmental risk. Nevertheless, cytotoxicity testing and first indicative results from a genotoxicity study extend present knowledge also with regard to possible effects on humans. The structural variability of commercially available ionic liquids as well as the abundance of theoretically accessible ionic liquids is illustrated and the consequences for an integrated risk assessment accompanying the development process are discussed. The side chain effect on toxicity for imidazolium type ionic liquids was confounded by more complex biological testing. Also, an influence of an anion on cytotoxicity is shown for the first time. Testing of presumed metabolites of the imidazolium type cations showed a significantly lower biological activity in cytotoxicity studies than their parent compounds. The importance of a purity assessment for ionic liquids is pointed out and a collection of methods that is believed to be adequate is presented. In addition to risk analysis, the use of life cycle analysis for the multi-objective problem of designing ionic liquids is sketched and an eco-design scheme for ionic liquids is proposed. In conclusion, the paper illustrates the complex nature of the development processes ionic liquids are currently undergoing and provides guidance on which aspects have to be kept in mind.

Effects of ionic liquids on the acetylcholinesterase – a structure–activity relationship consideration

Ionic liquids are discussed as sustainable green solvents, but toxicity and ecotoxicity data are rare. In this paper we present our results for different ionic liquids with the acetylcholinesterase inhibition assay. The results show that the acetylcholinesterase can be inhibited by ionic liquids containing a cation with a positively charged nitrogen and a certain lipophilicity. We tested imidazolium ionic liquids with different alkyl chains at R1 and R2 as well as with different anions and compared these results with our findings for other cation structures such as pyridinium ionic liquids and phosphonium ionic liquids. According to our results imidazolium and pyridinium ionic liquids inhibit the purified enzyme with EC50 values as low as 13 μM. The bulky phosphonium ionic liquids were less inhibitory. These results can be rationalized by structure–activity relationship considerations.

Qualitative and quantitative structure activity relationships for the inhibitory effects of cationic head groups, functionalised side chains and anions of ionic liquids on acetylcholinesterase

To contribute to a deeper insight into the hazard potential of ionic liquids to humans and the environment, an acetylcholinesterase (AchE) inhibition screening assay was used to identify toxicophore substructures and interaction potentials mediating enzyme inhibition.The positively charged nitrogen atom, a widely delocalised aromatic system, and the lipophilicity of the side chains connected to the cationic head groups can be identified as the key structural elements in binding to the enzymes active site. With respect to this, the dimethylaminopyridinium, the quinolinium and the pyridinium head groups exhibit a very strong inhibitory potential to the enzyme with IC50 values around 10 µM. In contrast, the polar and non-aromatic morpholinium head group is found to be only weakly inhibiting to the enzyme activity, with IC50 values > 500 µM.The introduction of polar hydroxy, ether or nitrile functions into the alkyl side chain is shown to be a potent structural alteration to shift the corresponding ionic liquids to a lower inhibitory potential. Supporting this fact, for a series of imidazolium cations, a QSAR correlation was set up by the linear regression of the log IC50versus the logarithm of the HPLC-derived lipophilicity parameter k0.Additionally, a broad set of anion species (inorganic, organic and complex borate anions), commonly used as ionic liquid counterions, was tested and the vast majority exhibited no effect on AchE. Only the fluoride and fluoride containing anion species which readily undergo hydrolytic cleavage can be identified to act as AchE inhibitors.

Reversed-phase liquid chromatographic method for the determination of selected room-temperature ionic liquid cations

The separation of selected 1-alkyl- and 1-aryl-3-methylimidazolium-based room temperature ionic liquid cations has been performed using reversed-phase high-performance liquid chromatography with electrospray ionization mass detection. The RP-HPLC method development started with the selection of a column taking into account especially the resolution of low molecular congeners of the selected group. Mobile phase composition was optimized for peak resolution, sensitivity and high reproducibility of retention values. The results of the method development were applied to the determination of exemplary ionic liquid species present in the medium used in cytotoxicity studies.

Enhanced photo-degradation of contaminants in petroleum refinery wastewater

In order to rise efficiency of the wastewater treatment in a refinery plant, several oxidation experiments were done, testing their applicability as an additional pretreatment method. The influence of treatment with low concentrations of H2O2 combined with stirring and UV light on degradation of organic compounds present in the refinery wastewater was studied. Oxidation of the total petroleum hydrocarbons occurs at relatively low concentrations of H2O2, additional UV irradiation slightly accelerates the process due to the increased formation of hydroxyl radicals. 1,2-dichloroethane and t-butyl methyl ether degrade in the similar manner and except for the lowest H2O2 concentration used (1.17 mM), the reduction after 24 h is total. The degradation rate for dichloromethane is the lowest one, depending both on hydrogen peroxide concentration and the presence of UV. Its maximum reduction of 83% was obtained using the highest applied peroxide concentration of 11.76 mM.