Measurement of 1-Octanol/Water Partition Coefficient of Surfactants By the Slow-Stirring Method

Abstract

The partition coefficient between 1-octanol and water (log P_ow) is a physicochemical property used to predict the bioaccumulation and toxicity of chemicals; however, an experimental method to measure the log P_ow of surfactants has yet to be established due to the problem of emulsion formation. It has been reported that the slow-stirring method can be used to measure the log P_ow of surfactants because it effectively avoids emulsion formation. However, a few studies have shown that the log P_ow of surfactants depends on initial concentration, even when it is below the critical micelle concentration (CMC). In this study, we investigated factors affecting log P_ow, such as initial concentration, using various surfactants. As a result, we found that the log P_ow of some surfactants was independent of concentration below the CMC. This indicates that optimizating the initial concentration based on the CMC is important when applying the slow-stirring method to surfactants. Notably, in one case, log P_ow decreased due to the formation of an O/W emulsion in the aqueous phase when the initial concentration was relatively high.