Critical Review and Interpretation of Environmental Data for Volatile Methylsiloxanes: Partition Properties

Abstract

Volatile methylsiloxanes (VMS) enter the environment through industrial activities and the use of various consumer products. Reliable measurements of environmental partition properties for these compounds are critical for accurate prediction of their environmental fate, distribution, transport, exposure and potential effects. In this study, the measured partition properties including air/water (K(AW)), octanol/water (K(OW)), and octanol/air partitioning coefficients (K(OA)), soil organic carbon/water distribution coefficient (K(OC)), and biological medium/fluid partition coefficients, and their temperature dependence were critically reviewed. Based on these results, organosilicon compounds such as methylsiloxanes are expected to behave differently in the environment compared to conventional hydrophobic environmental contaminants, as a result of their inherent characteristics related to molecular size and capacity for different types of molecular interactions that control partitioning. The differences are critical and need to be taken into consideration in environmental exposure and risk analyses of these compounds.

Fugacity and activity analysis of the bioaccumulation and environmental risks of decamethylcyclopentasiloxane (D5)

“… b The solubilities of D5 in water at nonstandard temperature were estimated using measured temperature dependence of the air–water partition coefficients and vapor pressure described in Xu et al .…”

“… d Octanol–water partition coefficients (Log K OW ) at different temperatures were derived from the empirical equation for the temperature dependence of log K OW : log K OW = 20.15 – 3596/ T , where T is temperature in Kelvin.…”

Decamethylcyclopentasiloxane (D5) environmental sources, fate, transport, and routes of exposure

“…Table provides a summary of the recommended physical and chemical properties of D5, including key intermedia partitioning and degradation properties (i.e., half‐lives). A more detailed discussion on methods for determining these properties and justification of these recommended properties and their temperature dependence is given in Kozerski et al , Xu and Kropscott , and Xu et al . In the present study, we focus on aspects of these properties that are of particular importance for evaluating fate and transport.…”

“…The present review specifically addresses steps 1 to 4 and step 7. Gobas et al address bioaccumulation (step 5), Fairbrother et al address toxicity and risk evaluation (steps 6 and 8), and Xu et al provide a more comprehensive treatment of physical and chemical properties (step 2).…”

Comment on “Unexpected Occurrence of Volatile Dimethylsiloxanes in Antarctic Soils, Vegetation, Phytoplankton, and Krill”

“…In addition, VMS have K AW and vapor pressure values greater than those of the known POPs by several orders of magnitude. 3 Volatilization of VMS from soil and water to the open air is expected to be rapid relative to the time scale of seasonal change in the Antarctic and the thawing of snow covered soil. In a simple equilibrium partitioning calculation considering soil with a depth of 10 cm and a column of air 100 m high, >97% of the VMS from the snowmelt should be re-volatilized to air.…”