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(M/MF180) Determination of hydrophobicity of surfactants by liposome-water partitioning for QSARs based on Kow. Costello, J.1, Marshall, S.1, 1 ABSTRACT- Predicting the aquatic toxicity of surfactants through the use of QSARs based on calculated octanol/water partition coefficients (Kowcalc) values has been criticised due to the inability to verify Kowcalc through measurement. This is because surfactants tend to aggregate at interfaces making it technically difficult to determine partitioning between bulk solvents. Muller et al. (Env. Tox. Chem. 18(10), 2191, 1999) used an alternative approach that overcomes the limitations of conventional octanol/water partitioning methods. Liposomes prepared from phosphatidylcholine and dispersed in a water phase occupy a glass half-cell which is separated from another half-cell, containing water only, by a dialysis membrane. Chemical is added to the water only cell and allowed to partition until steady state is achieved. We used this technique to measure the hydrophobicity of radio-labelled surfactants and non-surfactants. Firstly, experimental conditions were optimised (high lipid concentrations and low test substance concentrations were most favourable). Secondly, Klip of substances with different molecular shapes, i.e. high and low length:cross sectional area ratios were determined and compared with the respective Kows to assess whether the liposome technique could be used for a wide range of structures. Thirdly, the Klip of a range of surfactants was determined, including alcohol ethoxylate, linear alkylbenzene sulphonate and fatty acids. Finally, the Klip values for the surfactants were correlated with Kowcalc. Results of this study make a valuable contribution to the debate on whether Kowcalc represents an appropriate measure of surfactant hydrophobicity. Key words: hydrophobicity, liposome-water partitioning, surfactants, QSARs |
