The Influence of Varying Chain Lengths

All test systems showed higher toxicity with increasing n-alkyl chain length. 

Since ILs with longer alkyl chains possess more lipophilic properties [16], these results are in accordance with toxicity studies with other chemicals (e.g., Ref [18]). In addition the structures of the compounds investigated show similarity to cationic surfactants, especially imidazolinium compounds. Cationic surfactants are known to increase membrane permeability and therefore cause narcotic effects with increasing chain length [19]. 

Furthermore, for some cationic surfactants, quantitative structure-activity relationships have been established for daphnids and fish. Increments used in the ECOSAR toxicity estimation software (Syracuse Research Center, freely available from the US Environmental Protection Agency as part of the software EPI Suite) are -0.13 and -0.37 for each carbon atom in monoalkyl quaternary ammonium surfactants in daphnids and fish, respectively. These increments are much smaller than the ones found here. Our increments for the n-alkyl chain in Rj position are more in accordance with increments for CH2 groups in n-octanol/water partitioning constant estimation procedures, which are, e.g., 0.66 and 0.49 in Hansch and Leo s method and Meylan and Howard s method, respectively [20]. This suggests that uptake into the cells is governed by lipophilicity, or maybe more exactly by membrane/water partitioning [21, 22]. 

The dependence of toxicity on the chain length in the Rj position is clearly visible for all biological systems that have been tested up to now. 

Generally, ILs will be kept in more or less closed technical systems such as storage containers or batch reactors. It is clear that the risk for man and environment is connected with the possibility and magnitude of a release of the substance from these systems. The next factor determining the magnitude of the risk is the tendency 

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