Investigating hydrophilic and electrostatic properties of surfactants using retention on two mixed-mode liquid chromatographic columns.

In environmental risk assessment, it is essential to understand the relationship between molecular structure and fate and toxicity of organic contaminants. For surfactants, physico-chemical parameters which can reflect the interactions that determine surfactant behavior are not well defined and are therefore needed for the development of robust quantitative structure-activity relationships (QSAR). For the present study, we have measured HPLC retention times of several hydrocarbon and perfluorocarbon surfactant groups on a mixed-mode weak anion-exchange (WAX) and mixed-mode hydrophilic interaction liquid chromatography (HILIC) stationary phase. The nonionic alcohol ethoxylates are well retained on the HILIC column. Retention of anionic surfactants on the HILIC column is likely influenced by the degree of hydration of the surfactants and electrostatic repulsion from silanol groups. Less hydrated anionic surfactants (perfluoroalkyl carboxylates, perfluoroalkyl sulfonates and alkyl sulfates) show minimal hydrophilic interaction while other better hydrated anionic surfactants (alkyl carboxylates and alkyl sulfonates) are well retained. The retention mechanism of surfactants on both columns seems to be related to their degree of hydration, albeit expressed in different retention behavior: generally, retention on the WAX phase increases when retention on the HILIC phase decreases, and vice versa. The retention times from both columns were used to calculate retention factors (k') and these were subsequently used in calculating parameters that reflect the electrostatic property (kAX) and hydrophilic property (kHILIC) that determine the interaction between the hydrophilic part of the surfactant and the stationary phase. In further development of predictive models, we suggest the use of kAX for anionic surfactants and kHILIC for nonionic surfactants.