Evaluating the Salting-Out Effect on the Organic Carbon/Water Partition Ratios (KOC and KDOC) of Linear and Cyclic Volatile Methylsiloxanes: Measurements and Polyparameter Linear Free Energy Relationships

Dissolved inorganic salts influence the partitioning of organic chemicals between water and sorbents. We present new measurements of the salting-out constants (K-s) for partition ratios between water and organic carbon (K-OC) and between water and dissolved organic carbon (K-DOC) of three cyclic volatile methylsiloxanes (cVMS), two linear volatile methylsiloxanes (lVMS), three polychlorinated biphenyls (PCBs), and a-hexachlorocyclohexane (alpha-HCH). K-s, K-OC, and K-DOC were derived from volatilization rates of the chemicals from mixtures of water and organic carbon with varying concentrations of sodium chloride in a purge-and-trap system. K-OC and K-DOC values at different salinities were determined by fitting their values to reproduce observed volatilization rates using a fugacity-based multimedia model and assuming first-order kinetics for volatilization. The K-s values of cVMS and lVMS ranged from 0.160.76. The log K-OC of cVMS and lVMS in fresh water interpolated from our measurements ranged from 5.20 to 7.36 and the log K-DOC values from 5.04 to 6.72. Polyparameter linear free energy relationships (PP-LFERs) trained with data sets without measurements for siloxanes failed to accurately describe the log K-OC and log K-DOC of cVMS and lVMS. Including our measurements for cVMS and lVMS substantially improved the fit. PP-LFERs trained with data for K-s from solubility measurements do not describe our new measurements well regardless of whether or not they are included in the training set, which may reflect differences in the salting-out effect on partitioning to organic carbon versus on solubility.