Air-Water Partition Constants for Volatile Methyl Siloxanes

Abstract

Silicones are an important class of hydrophobic compounds in widespread use. To evaluate their fate in the environment, an accurate value of the air-water partition (Henry's law) constant is necessary, which, unfortunately, is lacking at present. A static head space and a newly developed dynamic vapor entry loop method were used to obtain the air-water partition constant for six volatile methyl siloxanes. Internally consistent data were obtained. The value of Henry's constant, as calculated from pure component vapor pressure and aqueous solubility, was 10- to 170-fold greater than the experimental values. A correction to the Henry's constant that involves the ratio of the thermodynamic activity coefficient for a methyl siloxane at infinite dilution to that at saturation solubility in the aqueous phase is proposed.

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

“…Kochetkov et al (2001) determined the Henry's law constant of D4 using two different methods. The first was a static method in which a saturated solution of D4 in water was equilibrated with air in the headspace of a sealed container for 48 hours and then the equilibrium concentration of D4 in each phase determined.…”

“…This value is self-consistent with the other partition coefficients used for D4. 5 To assess the sensitivity of the assessment to the Henry's law constant, and to reflect the uncertainty in the determination of this parameter, the effect of using a lower Henry's law constant of 60,060 Pa m 3 /mol at 28°C (equivalent to a dimensionless value of 24) on the conclusions of the assessment were also considered, based on the work of Kochetkov et al (2001). This sensitivity analysis is reported in Appendix A.…”

Methyl siloxanes in environmental matrices and human plasma/fat from both general industries and residential areas in China

“…Cyclic compounds (D4-D6) were detected in indoor air samples (n = 14) from these industrial facilities, with ∑ CMS ranging from 58.6 to 572 μg/m 3 (df = 100%). Although linear siloxanes had higher concentrations than cyclic compounds in most industrial additives/products, their total concentrations (0.05-11.2 μg/m 3 , df = 0-53%) in indoor air samples were much lower than those of cyclic siloxanes, which should be due to their lower vapor pressures (Kaj et al, 2005;Kochetkov et al, 2001). ∑CMS (D4-D6) in indoor dust samples (n = 14) from industrial facilities were 2.74-50.4 μg/g (df = 73-100%, Fig.…”

“…S2). This phenomenon could be explained by the following reasons: cyclic methyl siloxanes (D4-D6) with high vapor pressure (0.03-1.05 Torr) mainly exist in air, while linear compounds with low vapor pressure tend to exist in dust (Kaj et al, 2005;Kochetkov et al, 2001). In general, anti-dust mask could effectively reduce the exposure to pollutants via dust ingestion, but could not effectively reduce their exposure via inhalation.…”

Siloxanes Removal From Biogas Using Activated Carbon

“…Various types of volatile siloxanes have been discovered in biogas produced in the course of anaerobic digestion of organic materials, in higher concentrations than cyclic siloxanes with 4 and 5 structural units -SiO - [6].…”