Solubility Constraints on Aquatic Ecotoxicity Testing of Anionic Surfactants

In order to develop models that can predict the environmental behavior and effects of chemicals, reliable experimental data are needed. However, for anionic surfactants the number of ecotoxicity studies is still limited. The present study therefore aimed to determine the aquatic ecotoxicity of three classes of anionic surfactants. To this purpose we subjected daphnids (Daphnia magna) for 48 h to alkyl carboxylates (C_xCO₂^-), alkyl sulfonates (C_xSO₃^-), and alkyl sulfates (C_xSO₄^-) with different carbon chain lengths (x). However, all surfactants with x > 11 showed less than 50% immobility at water solubility. Hence, EC₅₀ values for only few surfactants could be gathered: C₉CO₂^- (16 mg L^-1), C₁₁CO₂^- (0.8 mg L^-1) and C₁₁SO₄^- (13.5 mg L^-1). Data from these compounds showed an increase in ecotoxicity with a factor 4.5 per addition of a hydrocarbon unit to the alkyl chain, and a factor 20 when replacing the sulfate head group by a carboxylate head group. Unfortunately, we could not test carboxylates with a broader variety of chain lengths because solubility limited the range of chain length that can be tested.