Assessing the factors responsible for ionic liquid toxicity to aquatic organisms via quantitative structure–property relationship modeling

Using previously published toxicity data as well as a small set of heretofore-unpublished results, quantitative structure–property relationship models are developed to assess the factors that govern the toxicity of a range of different ionic liquids to two aquatic organisms (Vibrio fischeri and Daphnia magna). With at most four molecular descriptors, log10 EC50 and log10 LC50 data are reproduced with an R2 of 0.78–0.88. Besides the well-established link between toxicity and alkyl chain length on imidazolium, pyridinium and quaternary ammonium-based ionic liquids, the models predict that toxicity increases slightly with the number of nitrogen atoms in an aromatic cation ring. All other things being equal, toxicity is expected to show the trend with cation type of ammonium < pyridinium < imidazolium < triazolium < tetrazolium. In addition, toxicity is expected to decrease with ring methylation as well as with an increase in the number of negatively charged atoms on the cation. The anion plays a secondary role in toxicity for the compounds studied here, although the presence of positively charged atoms on the anion are predicted to slightly increase toxicity.