Benefits from hazards: mixture hormesis induced by [emim]Cl despite its individual inhibitions

The threshold model based on monotonic concentration-response curves (CRCs) is unsuitable to assess the risk of chemicals with non-monotonic CRCs. The non-monotonic CRCs of mixtures may relate to the characteristics of some individual component. To reveal the cause of the mixtures resulting in the non-monotonic CRCs, we used the microplate toxicity analysis to determine the toxicity effects of six 1-alkyl-3-methyl-imidazolium ([amim]X) salts and their mixtures on Vibrio qinghaiensis sp.-Q67 (Q67). It was shown that the CRCs of six [amim]X salts are monotonic S-shaped while those of the senary mixtures designed by the uniform design ray (UD-ray) are all non-monotonic. The mixtures were further split into two ternary mixtures, one containing 1-ethyl-3-methyl-imidazolium ([emim]X) salts (noted as UTE) and the other one containing 1-butyl-3-methyl-imidazolium ([bmim]X) salts (noted as UTB). It was found that the CRCs of UTE mixtures are all non-monotonically J-shaped, while only one (UTB-R3) among UTB mixtures has a little stimulating effect and the CRCs of the other three mixtures (UTB-R1, UTB-R2 and UTB-R4) are monotonic. The CRCs of the binary mixtures designed by the direct equipartition ray design (EquRay) procedure were further examined. The CRCs of the mixtures containing [emim]Cl are non-monotonic J-shaped while those of the mixtures without [emim]Cl are still monotonic. Thus, it can conclude that it is [emim]Cl that causes the non-monotonic CRCs in [amim]X mixtures, even though the CRC of individual [emim]Cl is monotonic.