Determination of polydimethylsiloxane-to-air and polydimethylsiloxane-to-water partition coefficients of octamethylcyclotetrasiloxane and potential implications for polymer aquatic hazard assessment

Octamethylcyclotetrasiloxane (D4) is a cyclic volatile methylsiloxane compound associated with the production of polydimethylsiloxanes (PDMS). Depending on processing conditions, silicone fluids made by equilibration polymerization can contain residual D4 at parts-per-million to parts-per-hundred levels. When silicone fluids enter the environment through use or disposal, aquatic organisms may be exposed to residual D4. To accurately assess the contribution of D4 to the aquatic hazard of silicone fluids, knowledge of the partitioning behavior of D4 is needed. In this study, PDMS-to-air partition coefficients (KPDMS-air) for D4 were directly measured at 21 °C using a static equilibration method. The influence of various factors on KPDMS-air was explored, including the PDMS fluid viscosity (molecular weight), the D4 concentration in the polymer, addition of hydrophobized fumed silica to PDMS, and the presence of amine functional groups within the PDMS structure. For permethylated PDMS, log KPDMS-air values varied between 4.39 and 4.53. Incorporation of treated silica filler at up to 25%w/w had no impact on KPDMS-air, while introduction of (aminopropyl)methyl-dimethylsiloxy units to the polymer (3.7 wt% nitrogen) produced modestly lower log KPDMS-air, 4.19. The KPDMS-air values were combined with reported air-water partition coefficients (Kair-water) for D4 to calculate values of KPDMS-water based on the thermodynamic cycle. The resulting log KPDMS-water values ranged from 6.88 to 7.22, which were used to estimate the maximum attainable aqueous D4 concentrations for different polymer/water phase ratios. For a D4 content of 0.025% w/w in the polymer, the current threshold for classification of the polymer as hazardous to the aquatic environment, the maximum aqueous D4 concentration was ≤ 0.032 μg/L. These concentrations were at least 100-fold less than chronic aquatic toxicity thresholds derived from studies with D4, suggesting that the 0.025% w/w threshold is overly stringent in assigning aquatic hazard classifications to PDMS materials like those tested here.