A novel method for measuring polymer–water partition coefficients

Abstract

Low density polyethylene (LDPE) often is used as the sorbent material in passive sampling devices to estimate the average temporal chemical concentration in water bodies or sediment pore water. To calculate water phase chemical concentrations from LDPE concentrations accurately, it is necessary to know the LDPE–water partition coefficients (KPE–w) of the chemicals of interest. However, even moderately hydrophobic chemicals have large KPE–w values, making direct measurement experimentally difficult. In this study we evaluated a simple three phase system from which KPE–w can be determined easily and accurately. In the method, chemical equilibrium distribution between LDPE and a surfactant micelle pseudo-phase is measured, with the ratio of these concentrations equal to the LDPE–micelle partition coefficient (KPE–mic). By employing sufficient mass of polymer and surfactant (Brij 30), the mass of chemical in the water phase remains negligible, albeit in equilibrium. In parallel, the micelle–water partition coefficient (Kmic–w) is determined experimentally. KPE–w is the product of KPE–mic and Kmic–w. The method was applied to measure values of KPE–w for 17 polycyclic aromatic hydrocarbons, 37 polychlorinated biphenyls, and 9 polybrominated diphenylethers. These values were compared to literature values. Mass fraction-based chemical activity coefficients (γ) were determined in each phase and showed that for each chemical, the micelles and LDPE had nearly identical affinity.