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.

Solubility Constraints on Aquatic Ecotoxicity Testing of Anionic Surfactants.

In order to develop models that can predict the environmental behavior and effects of chemicals, reliable experimental data are needed. However, for anionic surfactants the number of ecotoxicity studies is still limited. The present study therefore aimed to determine the aquatic ecotoxicity of three classes of anionic surfactants. To this purpose we subjected daphnids (Daphnia magna) for 48 h to alkyl carboxylates (CxCO2-), alkyl sulfonates (CxSO3-), and alkyl sulfates (CxSO4-) with different carbon chain lengths (x). However, all surfactants with x > 11 showed less than 50% immobility at water solubility. Hence, EC50 values for only few surfactants could be gathered: C9CO2- (16 mg L-1), C11CO2- (0.8 mg L-1) and C11SO4- (13.5 mg L-1). Data from these compounds showed an increase in ecotoxicity with a factor 4.5 per addition of a hydrocarbon unit to the alkyl chain, and a factor 20 when replacing the sulfate head group by a carboxylate head group. Unfortunately, we could not test carboxylates with a broader variety of chain lengths because solubility limited the range of chain length that can be tested.

Molecular simulation of polycyclic aromatic hydrocarbon sorption to black carbon.

Strong sorption of hydrophobic organic contaminants to soot or black carbon (BC) is an important environmental process limiting the bioremediation potential of contaminated soils and sediments. Reliable methods to predict BC sorption coefficients for organic contaminants are therefore required. A computer simulation based on molecular mechanics using force field methods has been applied in this study to calculate BC sorption coefficients of polycyclic aromatic hydrocarbons (PAHs). The free energy difference between PAHs dissolved in water and in water containing a model structure of BC was calculated by thermodynamic integration of Monte Carlo simulated energies of transfer. The free energies were calculated with a hypothetical reference state that has equal free energies in both phases and is therefore cancelled in the calculated free energy difference. The calculated sorption coefficient of phenanthrene (log K(BC) = 5.17 +/- 0.54 L/kg C), fluoranthene (6.33 +/- 0.64 L/kg C) and benzo[a]pyrene (7.38 +/- 0.36 L/kg C) corresponded very well to experimental values available in the literature. Furthermore, an average spacing distance of 3.73 A between PAHs and BC was determined that is only slightly lower than an experimentally determined value of 4.1 A. The method applied in this study enables the calculation of the extent of PAH sorption to a soot surface for which no experimental values are available nor data for related compounds as required in quantitative structure-activity relationships.