Cyclic volatile methyl siloxanes (cVMS) such as octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) may enter the environment through industrial activities and the use of various consumer products. Reliable air/water (K(AW)), 1-octanol/water (K(OW)), and octanol/air partition coefficients (K(OA)) for those compounds and their common degradation product, dimethylsilanediol, are critical for accurate prediction of the environmental fate, distribution, and transport of these materials. Challenges have been encountered in determining these properties for cVMS and their degradation products mainly due to the extremely low water solubility of the organosiloxanes, low volatility of their degradation products, and reactivity of those compounds in the water/1-octanol system that can lead to inconsistent and inaccurate partition coefficients. A novel direct method is presented for the simultaneous determination of K(AW), K(OW), and K(OA) of organic compounds and was applied to these organosilicon compounds. It was tested in a range of log K(AW) values from -6.8 to 3.1, log K(OW) values from -0.4 to 8.9, and log K(OA) values up to 7. The advantages of the new direct method include the improved accuracy, a shortened measurement time, simultaneous measurement of three partition coefficients of multiple compounds, self-consistency among resultant partition coefficients, and a wide range of applicability including materials that may be slowly reactive in the water/1-octanol system.

Partitioning equilibria and their temperature dependence of chemicals between different environmental media are important in determining the fate, transport, and distribution of contaminants. Unfortunately, internally consistent air/water (KAW), 1-octanol/air (KOA), and 1-octanol/water (KOW) partition coefficients, as well as information on their temperature dependence, are scarce for organosilicon compounds because of the reactivity of these compounds in water and octanol and their extreme partition coefficients. A newly published 3-phase equilibrium method was evaluated for simultaneous determination of the temperature dependence of KAW, KOA, and KOW of 5 volatile methylsiloxanes (VMS) and trimethylsilanol (TMS) in a temperature range from 4 °C to 35 °C. The measured partition coefficients at the different temperatures for any given compound, and the enthalpy and entropy changes for the corresponding partition processes, were all internally consistent, suggesting that the 3-phase equilibrium method is suitable for this type of measurement. Compared with common environmental contaminants reported in the literature, VMS have enthalpy and entropy relationships similar to those of alkanes for air/water partitioning and similar to those of polyfluorinated compounds for octanol/air partitioning, but more like those for benzoates and phenolic compounds for octanol/water partitioning. The temperature dependence of the partition coefficients of TMS is different from those of VMS and is more like that of alcohols, phenols, and sulfonamides. Environ Toxicol Chem 2014;33:2702–2710. © 2014 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc.

The octanol/air partition coefficient (K OA ) is a key parameter used to predict the long-range transport potential (LRTP) of volatile and semivolatile organic compounds and their bioaccumulation in terrestrial biota. Despite the enormous importance of this parameter, reliable K OA values are not available for volatile methylsiloxanes (VMS). In this study, a method of syringe headspace analysis was developed to determine K OA values for six common VMS at trace levels. It was found that log K OA values of any given VMS were linearly related to the reciprocal of the environmental temperature (T) from −3°C to 40°C, whereas at any given T, the log K OA values of different VMS were linearly related to their molecular mass and normal boiling point temperatures (T b ). Based on those findings, empirical models were developed to predict log K OA values of methylsiloxanes using T b (or molecular mass) and T as the only independent variables. The log K OA values for common VMS at 25°C were in the range from 2.98 (for hexamethyldisiloxane) to 5.77 (for dodecamethylcyclohexasiloxane). Judging from the log K OA values, these VMS have little potential for long-range transport and deposition to remote surface media and for bioaccumulation in terrestrial biota.