Determination of soil‐water sorption coefficients of volatile methyl siloxanes
The sorption behaviors of four cyclic and linear volatile methyl siloxane (VMS) compounds between water and organic matter in three United Kingdom soils were studied by a batch equilibrium method using 13C‐enriched sorbates. Sorption and desorption kinetics and isotherms were determined for octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), octamethyltrisiloxane (L3) and decamethyltetrasiloxane (L4). Concentrations of [13C]‐VMS in the soil and aqueous phases were measured directly by extraction/GC‐MS techniques. All VMS were sorbed rapidly, reaching constant distributions in all soils by 24 h. Desorption kinetics were very rapid, with re‐attainment of equilibrium within 1 h. In the main, linear isotherms were observed for aqueous concentrations at or below 4 % of the solubility limits. The average sorption log KOC values across soils were 4.23 for D4, 5.17 for D5, 4.32 for L3, and 5.13 for L4, with standard deviations of 0.09 to 0.34. Desorption KOC values were systematically greater by 0.1 to 0.3 log units. The linear isotherms and low variation in KOC values across soils suggested partitioning‐dominated sorption of the VMS. Compared with traditional hydrophobic organic compounds, KOC values for the VMS were significantly lower than expected on the basis of their octanol‐ water partition coefficients. A linear free energy relationship analysis showed that these differences could be rationalized quantitatively in terms of the inherent characteristics of the VMS, combined with the differences in solvation properties of organic matter and octanol.