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

Abstract

Knowing the temperature dependence of the organic carbon/water partition ratios (K OC ) of volatile methylsiloxanes (VMS) is required to understand their environmental fate. We measured the K OC 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 (ΔH OC and ΔS OC , respectively). The ΔH OC of VMS ranged from −79.2 to −45.8 kJ mol −1 , while the ΔH OC 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 (ΔH OW ) for cVMS (11.3−68.8 kJ mol −1 ) differed substantially from our ΔH OC measurements, even showing different signs (negative versus positive). Literature data of ΔH OC and ΔH OW for PCBs (−61 to −17 kJ mol −1 ) are closer to our measured values of ΔH OC 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 ΔH OC and ΔH OW , especially for the VMS. Therefore, assuming ΔH OC equals ΔH OW in environmental fate models may be a source of substantial error.

A critical assessment of the environmental fate of linear and cyclic volatile methylsiloxanes using multimedia fugacity models

“…Environmental Science: Processes & Impacts Paper scenarios using the K OC measurements of Kozerski et al, 26 the residence times were all shorter than in scenarios using the measurements of Panagopoulos et al 16,17 and in the case where we used the DH OW measurements of Xu and Kropscott, 31 the residence times were very close to the criterion values for persistence in marine sediments. On the other hand, scenarios using the DH OC measurements of Panagopoulos et al 32 had residence times of VMS that exceeded the REACH criterion for marine sediments in all cases except for D 4 , where residence times were almost equal to the residence time of the REACH criterion. 24 Similar results were observed for the overall residence times.…”

“…[27][28][29][30] Xu and Kropscott 31 studied the effect of temperature on the partition ratios of VMS between octanol and water (K OW ) and observed that the K OW of VMS decreased with decreasing temperature. In another recent study, 32 we measured the effect of temperature on the K OC of VMS and observed that K OC increased with decreasing temperatures. This difference could result in substantial differences in the modeled environmental fate and the residence times of VMS in aquatic environments at temperatures lower than the reference temperature of K OC measurements.…”

“…31 The values for DH OW , DH AW and DH OA of D 6 and L 5 were calculated by linearly extrapolating from the measured values based on the chemicals' molecular weight. e The values for DH OC were measured by Panagopoulos et al 32 f The activation energies (E act ) for degradation in the air of D 4 and D 5 were measured in the study of Xiao et al 13 The value of D 6 was calculated by extrapolating from the data for D 4 and D 5 . The value for L 4 was measured by Zammit 38 and the value of L 5 was calculated by extrapolating from the data of Zammit 38 for L 2 , L 3 and L 4 .…”

“…3 are for K OC corrected for temperature using the DH OW measurements of Xu and Kropscott 31 and results in Fig. 4 are for a scenario in which K OC was corrected for temperature changes using the DH OC measurements of Panagopoulos et al 32 We present both compartment-specic and overall residence times.…”

“…4 Water, sediment and overall residence times for cVMS (circles) and lVMS (squares) using both the K OC measurements of Kozerski et al 26 (lower panels) and those of Panagopoulos et al 16,17 (upper panels). The K OC was corrected for temperature changes using the DH OC measurements of Panagopoulos et al 32 The blue line shows the REACH criterion for persistence in marine waters and the green line shows the REACH criterion for persistence in marine sediments. 24 The red line shows the 100-day overall persistence criterion suggested by Webster et al 25 190 | Environ.…”