Comprehensive Analysis of the Acute Toxicity of Ionic Liquids Using Microtox® Bioassays

The ecotoxicity of a set of 30 ionic liquids, namely 23 aprotic compounds (APILs) and 7 protic compounds (PILs), was analyzed in this work by monitoring the inhibition of the bioluminescence of the bacteria Aliivibrio fischeri with varying concentrations of ILs utilizing the Microtox® standard toxicity test. The study covered ILs that have various synthetic natures, PILs and APILs, with a common anion or cation, and different alkyl chain lengths. The results indicate that both moieties, anion and cation, have an influence on toxicity, these being the ILs with the bis((trifluoromethyl)sulfonyl)imide (TFSI) anion and imidazolium cation, which are the most harmful, while those less toxic correspond to ammonium-based ILs. The alkyl chain length seems to have the most significant impact on toxicity, except for tris(pentafluoroethyl) trifluorophosphate (FAP) anion-based ILs, which, interestingly, showed the opposite behavior. A critical alkyl size (CAS) at C = 6 was observed for the rest of the families, resulting in a significant reduction in the effective concentration (EC) values: the connection between this CAS and toxicity has never been observed before, and it indicates a threshold that marks the end of harmlessness (C < 6) and the start of toxicity (C > 6).