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Temperature Dependence of the Organic Carbon/Water Partition Ratios (KOC) of Volatile Methylsiloxanes

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journal contribution
posted on 12.05.2017, 00:00 by Dimitri Panagopoulos, Annika Jahnke, Amelie Kierkegaard, Matthew MacLeod
Knowing the temperature dependence of the organic carbon/water partition ratios (KOC) of volatile methylsiloxanes (VMS) is required to understand their environmental fate. We measured the KOC of two linear VMS (lVMS), three cyclic VMS (cVMS), and six polychlorinated biphenyls (PCBs) at 25, 15, 10, and 5 °C and calculated their enthalpies and entropies of sorption to organic carbon (ΔHOC and ΔSOC, respectively). The ΔHOC of VMS ranged from −79.2 to −45.8 kJ mol–1, while the ΔHOC of the PCBs ranged from −68.7 to −29.3 kJ mol–1. Previously reported measurements of the enthalpy of phase change between octanol and water (ΔHOW) for cVMS (11.3–68.8 kJ mol–1) differed substantially from our ΔHOC measurements, even showing different signs (negative versus positive). Literature data of ΔHOC and ΔHOW for PCBs (−61 to −17 kJ mol–1) are closer to our measured values of ΔHOC for the PCBs showing the same sign (negative) with differences within a factor of 2 in the majority of the cases. Comparison of all available data for PCBs and VMS indicated that there may be important differences between ΔHOC and ΔHOW, especially for the VMS. Therefore, assuming ΔHOC equals ΔHOW in environmental fate models may be a source of substantial error.