Aquatic toxicity of ethoxylated and propoxylated alcohols to daphnia magna.

Ethoxylated alcohols, which are used as nonionic surfactants, are known to act as general narcotics in acute aquatic toxicity; that is, they behave in the same way as nonsurfactant unreactive organic chemicals. The toxicity of such chemicals is well predicted by quantitative structure-activity relationships based solely on the logarithm of the octanol/water partition coefficient (log P), which can be calculated from structure. In the present study, we have shown, using experimental results, that a similar approach can be used to determine the toxicity of ethoxylate/propoxylate alcohols (i.e., containing propoxy [PO] and ethoxy [EO] units). Our calculations indicate that use of the Roberts position-dependent branching factor in calculating the PO group contribution is more appropriate than the Leo and Hansch branch factor. The resulting log P value for a PO group is 0.01; that is, the overall contribution to the final log P value is close to zero. On this basis, it is predicted that nonionic surfactants containing both EO and PO groups should have the same molar toxicity as surfactants based on the same parent alcohol and with the same number of EO groups but with no PO groups. This prediction has been confirmed in Daphnia acute toxicity tests. Furthermore, both EO/PO and EO-only nonionics are found to fit the same linear relationship between log P and toxicity.