A comparison of log Kow (n-octanol-water partition coefficient) values for non-ionic, anionic, cationic and amphoteric surfactants determined using predictions and experimental methods

BackgroundSurfactants are widely used across the globe both in industrial and consumer products. The n-octanol/water partition ratio or coefficient (log K-ow) and n-octanol/water distribution coefficient (log D) are key parameters in environmental risk assessment of chemicals as they are often used to estimate the environmental fate and bioavailability and thus exposure and toxicity of a compound. Determining log K-ow data for surfactants is a technical challenge due to their amphiphilic properties. Currently several existing experimental OECD methods (e.g. slow-stirring, HPLC, solubility ratio) and QSPR models are available for log K-ow/D measurement or prediction. However, there are concerns that these methods have not been fully validated for surfactants and may not be applicable due to the specific phase behaviour of surfactants.ResultsThe current methods were evaluated for the four surfactant classes (non-ionic, anionic, cationic and amphoteric). The solubility ratio approach, based on comparative n-octanol and water solubility measurements, did not generate robust or accurate data. The HPLC method generates consistently higher log K-ow values than the slow-stirring method for non-ionics, but this positive bias could be removed using reference surfactants with log K-ow values determined using the slow-stirring method. The slow-stirring method is the most widely applicable experimental method for generating log K-ow/D data for all the surface-active test compounds. Generally, QSPR-predicted log K-ow/D values do not correlate well with experimental values, apart for the group of non-ionic surfactants. Relatively, large differences in predicted log K-ow/D values were observed when comparing various QSPR models, which were most noticeable for the ionised surfactants.ConclusionsThe slow-stirring method is the most widely applicable experimental method for generating log K-ow/D data for all the four surfactant classes. A weight of evidence approach is considered appropriate for non-ionic surfactants using experimental and model predications. However, it is more difficult to apply this approach to ionisable surfactants. Recommendations are made for the preferred existing QSPR predictive methods for determination of log K-ow/D values for the surfactant classes. Investigation of newer alternative experimental log K-ow methods as well as more biologically relevant and methodologically defensible alternative methods for describing partitioning of surfactants are recommended.