QSAR and mechanism of action for aquatic toxicity of cationic surfactants

This paper considers the mechanism of aquatic toxicity of cationic organic compounds and the relationship of toxicity with log P (P = octanol/water partition coefficient). The rather complex log P calculation method given by Leo and Hansch for cationics is reconsidered and the underlying solvation chemistry is reinterpreted. This reinterpretation is tested by application to micellisation data for cationic surfactants. It is found that in its micellisation potential, expressed as pCMC, a cationic surfactant is similar to the anionic surfactant with the same hydrophobe. Our modifications, derived from this finding, to the log P calculation for cationics, lead to a cationics fish toxicity QSAR whose slope and intercept are close to the range commonly found for polar narcosis QSARs. The compounds included in this QSAR include 2 non-surfactants, indicating that for cationics, as already observed for anionics and nonionics, whether a compound is a surfactant or not is irrelevant to its toxicity. That the cationics act by a polar narcosis mechanism is supported by the results of mixture toxicity studies, indicating additive joint action for mixtures of a cationic with known polar narcotics and independent joint action for a mixture of a cationic with a known general narcotic.