Intriguingly, for AB0.2, a noticeable increase in acute toxicity toward C. vulgaris compared with ILs A and B was observed. The determined EC50 value was 0.88 mg•dm -3 which corresponds to 1.56 µmol‧dm -3 . Considering the mechanism of surfactant toxicity against algal cells, the increased ecotoxicity of AB0.2 was most likely due to the previously described surface activity enhancement, which causes the DSILs to have lower CMCs than their parent ILs do. The effect of the surface activity of a given surfactant on its toxicity to aquatic organisms can be expressed in terms of the CMC/EC50 quotient. The values of this quotient for A, B, and AB0.2 ranged from 91 (A) to 142 (AB0.2) and differed by less than one order of magnitude (see Table .16, Supplementary Data). This means that the toxicity of the tested ILs and DSILs in aqueous solutions increases as the CMC value of a given surfactant decreases, and this relationship is similar to that of another group of cationic surfactants described previouslyalkyl betainate bromides and alkyl dibetainate dibromides . However, compared with conventional quaternary ammonium salts with a substituent of the same length, namely, cetyltrimethylammonium chloride (CTAC, EC50: 0.14 mg•dm -3 ) or benzyldimethylhexadecylammonium chloride (BAC16, EC50: 0.16 mg•dm -3 ), the DSIL AB0.2 is less toxic to C. vulgaris . This type of activity also indicates additional potential for the use of AB0.2 as an algicide (e.g., to counteract algal growth in pools or ponds) at well-defined concentrations. Algicides are also used to counteract eutrophication in natural water bodies.