title: Evaluation of the three-phase equilibrium method for measuring temperature dependence of internally consistent partition coefficients (K(OW), K(OA), and K(AW)) for volatile methylsiloxanes and trimethylsilanol.
abstract: 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 (K(AW)), 1-octanol/air (K(OA)), and 1-octanol/water (K(OW)) 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 (K(OW), K(OA), and K(AW)) 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.
authors.affiliation: Health and Environmental Sciences, Dow Corning, Auburn, Michigan, USA.
authors.forename: Shihe
authors.initials: S
authors.lastname: Xu
authors.forename: Bruce
authors.initials: B
authors.lastname: Kropscott
mesh_terms: D020003:1-Octanol / Q000737:chemistry; D000388:Air; D002851:Chromatography, High Pressure Liquid; D019277:Entropy; D007700:Kinetics; D012833:Siloxanes / Q000737:chemistry*; D013696:Temperature; D014297:Trimethylsilyl Compounds / Q000737:chemistry*; D055549:Volatile Organic Compounds / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D012833:Siloxanes; D014297:Trimethylsilyl Compounds; D055549:Volatile Organic Compounds; D014867:Water; D020003:1-Octanol; C055926:trimethylsilanol
keywords: Enthalpy and entropy changes; Enthalpy and entropy compensation effects; Partition coefficients; Temperature dependence; Volatile methylsiloxanes