Experimental Solubility Approach to Determine PDMS–Water Partition Constants and PDMS Activity Coefficients

Freely dissolved aqueous concentration and chemical activity are important determinants of contaminant transport, fate, and toxic potential. Both parameters are commonly quantified using Solid Phase Micro-Extraction (SPME) based on a sorptive polymer such as polydimethylsiloxane (PDMS). This method requires the PDMS–water partition constants, <i>K</i>_PDMSw, or activity coefficient to be known. For superhydrophobic contaminants (log <i>K</i>_OW >6), application of existing methods to measure these parameters is challenging, and independent measures to validate <i>K</i>_PDMSw values would be beneficial. We developed a simple, rapid method to directly measure PDMS solubilities of solid contaminants, <i>S</i>_PDMS(S), which together with literature thermodynamic properties was then used to estimate <i>K</i>_PDMSw and activity coefficients in PDMS. PDMS solubility for the test compounds (log <i>K</i>_OW 7.2–8.3) ranged over 3 orders of magnitude (4.1–5700 μM), and was dependent on compound class. For polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-<i>p</i>-dioxins (PCDDs), solubility-derived <i>K</i>_PDMSw increased linearly with hydrophobicity, consistent with trends previously reported for less chlorinated congeners. In contrast, subcooled liquid PDMS solubilities, <i>S</i>_PDMS(L), were approximately constant within a compound class. <i>S</i>_PDMS(S) and <i>K</i>_PDMSw can therefore be predicted for a compound class with reasonable robustness based solely on the class-specific <i>S</i>_PDMS(L) and a particular congener’s entropy of fusion, melting point, and aqueous solubility