QSAR studies of surfactant toxicity to Daphnia magna.
The inherent nature of surfactants to aggregate at surfaces makes
measurement of log P (octanoll water partition coefficient) for these substances
extremely difficult. It is possible, however, to calculate a log P descriptor based on
the method described by Hansch and Leo (1979).
Work presented in this thesis describes the study of the acute toxicity of
sulphonated esters (FAES) of general formula R-CH(S03"Na +)-C02-R' to Daphnia
magna. Due to structural similarities of this class of anionic surfactant to linear
alkylbenzene sulphonate (LAS), it was considered that the log P based QSAR
originally developed to describe the toxicity of LAS to D. magna (Roberts, 1989)
also would be a good predictor of the acute toxicity for FAES substances.
Results of the toxicity studies showed that FAES substances were less toxic
than predicted. However, when plotted against log P' Calculated using the
conventional fragment approach of Hansch and Leo with the addition of a position
dependent branching factor (PDBF) to account for water sharing between
hydrocarbon chains, the regression slope was para"el to but distinct from that of
LAS. This indicated that either FAES substances were not acting as by the same
mode of action as LAS or that modification of the log P calculation was required.
Further studies of the toxicity of binary mixtures of FAES with known polar
and non-polar narcotics, established that FAES exhibited concentration addition
with LAS and phenol. This indicated that they behaved with a similar mode of
action and it would be expected that LAS and FAES would share the same QSAR.
The difference of the regression slopes of FAES and LAS observed· earlier,
therefore, suggested the requirement of a modification to the original log P
The modified proximity factor developed in this thesis considers the effects
of relative size of proximal polar fragments on log P.· Spherical hydration sheaths
surrounding each fragment were assumed and 'overlapping volumes calculated for
fragments at different carbon separation. When incorporated into the log P
calculation, the new log P values now allow toxicity values for LAS and F AES
substances to be incorporated into the same QSAR