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 (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.

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

“…Values of log K OA at room temperature using 3-phase equilibrium 12,17 are almost identical to those determined with the commercial 1-octanol phase (non-water saturated) 31 (Table 5).…”

“…However, in the real environment with varying temperatures, the temperature dependence of the partition coefficients is also an important consideration. In Figure 2, logarithmic values of the three partition coefficients by 3-phase methods 12,17 were plotted together with the log K OA values by the syringe method 31 for selected VMS. As shown in Figure 2, temperature does have substantial effects on the partition coefficients of VMS, especially log K AW and log K OA values.…”

“…This mutual solubilization effect, plus the cumulative error from separate measurement of each partition coefficient, makes internal consistency difficult to achieve. 12,17 As shown in Table 6, the three partition coefficients measured by those approaches do not achieve internal consistency defined in eq 10. The discrepancy of the partition coefficients, δ, defined as the difference between log K OW and the sum of log K AW and log K OA at the same temperature (i.e., δ = log K OW − (log K AW + 17 and evaluated 12,17 recently for simultaneous determination of three partition coefficients involving two liquid phases and air.…”

“…12,17 As shown in Table 6, the three partition coefficients measured by those approaches do not achieve internal consistency defined in eq 10. The discrepancy of the partition coefficients, δ, defined as the difference between log K OW and the sum of log K AW and log K OA at the same temperature (i.e., δ = log K OW − (log K AW + 17 and evaluated 12,17 recently for simultaneous determination of three partition coefficients involving two liquid phases and air. Briefly, a test compound is introduced to a double-syringe apparatus for equilibration in water, 1-octanol and air phases simultaneously (Figure 1), and K AW , K OA , and K OW are obtained from measured concentrations of the test compound in the three phases.…”

“…Log K OW values of VMS were determined for several VMS, 26 which gave much smaller values than those obtained recently. 9,12,17 A comprehensive understanding of the methods employed by Bruggeman et al 26 is complicated by the absence of experimental details in the report. However, there are two potential issues with this HPLC retention time correlation method, which render the log K OW values for VMS suspect.…”