Fragment-based approach to calculate hydrophobicity of anionic and nonionic surfactants from chromatographic retention on a C₁₈ stationary phase

To predict the fate and potential effects of organic contaminants, information about their hydrophobicity is required. However, common parameters to describe the hydrophobicity of organic compounds (e.g., octanol–water partition constant [K_OW]) proved to be inadequate for ionic and nonionic surfactants because of their surface-active properties. As an alternative approach to determine their hydrophobicity, the aim of the present study was therefore to measure the retention of a wide range of surfactants on a C₁₈ stationary phase. Capacity factors in pure water (k′₀) increased linearly with increasing number of carbon atoms in the surfactant structure. Fragment contribution values were determined for each structural unit with multilinear regression, and the results were consistent with the expected influence of these fragments on the hydrophobicity of surfactants. Capacity factors of reference compounds and log K_OW values from the literature were used to estimate log K_OW values for surfactants (log K_OW^HPLC). These log K_OW^HPLC values were also compared to log K_OW values calculated with 4 computational programs: KOWWIN, Marvin calculator, SPARC, and COSMOThermX. In conclusion, capacity factors from a C₁₈ stationary phase are found to better reflect hydrophobicity of surfactants than their K_OW values.