Ecotoxicity profiling of a library of 24 l-phenylalanine derived surface-active ionic liquids (SAILs)

Abstract

We evaluated the ecotoxicity of a library of 24 l-phenylalanine derived surface-active ionic liquids (SAILs) with various cationic head groups (pyridinium, Py; imidazolium, Imid and cholinium, Chol) and alkyl ester chains from C2 to C16. For toxicity evaluation we used 72-h algal growth inhibition assay (OECD 201) with Raphidocelis subcapitata and 24-h mortality test with aquatic crustaceans Thamnocephalus platyurus (ISO 14380:2011). The OECD 201 assay was applied to all 24 SAILs while the ISO 14380:2011 test was applied to a subset, specifically all eight pyridinium SAILs and C6 and C8 examples of the imidazolium and cholinium SAILs (total 12 SAILs). For the comparison, 30-min EC50 data (based on inhibition of bioluminescence) previously reported by this group for the 24 SAILs for marine bacteria Vibrio fischeri (ISO 21338:2010) were included and correlated to the algae and aquatic crustaceans data. According to the results of the multritrophic test battery only two studied SAILs - PyC2 and CholC2 – could be considered ‘low toxicity’, (i.e. were ranked not harmful, L(E)C50 > 100 mg/L by the most sensitive test - algal growth inhibition assay). T. platyurus proved about 100-times more tolerant to studied SAILs than algae. An alternative classification scale dependent on the average MW of the compound dataset (based on molar concentrations and not concentrations based on mg/L) was suggested to rank the compounds. When compared to the classification scale independent of the MW of the compound, a more accurate appraisal was achieved for suggesting the greener alternatives for certain commercial SAILs/surfactants.