pH-Dependent Partitioning of Ionizable Organic Chemicals between the Silicone Polymer Polydimethylsiloxane (PDMS) and Water

Abstract

The silicone polymer polydimethysiloxane (PDMS) is a popular passive sampler for in situ and ex situ sampling of hydrophobic organic chemicals. Despite its limited sorptive capacity for polar and ionizable organic chemicals (IOC), IOCs have been found in PDMS when extracting sediment and suspended particulate matter. The pH-dependent partitioning of 190 organics and IOCs covering a range of octanol−water partition constants log K ow from −0.3 to 7.7 was evaluated with a 10-day shaking method using mixtures composed of all chemicals at varying ratios of mass of PDMS to volume of water. This method reproduced the PDMS−water partition constant K PDMS/w of neutral chemicals from the literature and extended the dataset by 93 neutral chemicals. The existing quantitative structure−activity relationship between the log K ow and K PDMS/w could be extended with the measured K PDMS/w linearly to a log K ow of −0.3. Fully charged organics were not taken up into PDMS. Thirty-eight monoprotic organic acids and 42 bases showed negligible uptake of the charged species, and the pH dependence of the apparent D PDMS/w (pH) could be explained by the fraction of neutral species multiplied by the K PDMS/w of the neutral species of these IOCs. Seventeen multiprotic chemicals with up to three acidity constants pK a also showed a pH dependence of D PDMS/w (pH) with the tendency that the neutral and zwitterionic forms showed the highest D PDMS/w (pH). D PDMS/w (pH) of charged species of more hydrophobic multiprotic chemicals such as tetrabromobisphenol A and telmisartan was smaller but not negligible. Since these chemicals show high bioactivity, their contribution to mixture effects has to be considered when testing passive sampling extracts with in vitro bioassays. This work has further implications for understanding the role of microplastic as a vector for organic micropollutants.

Critical Assessment of pH‐Dependent Lipophilicity Profiles of Small Molecules: Which One Should We Use and In Which Cases?

“…Additionally, in environmental chemistry research, passive equilibrium sampling of dissolved contaminants in water has been studied using polymer polydimethylsiloxane (PDMS) as an absorbent phase. This hydrophobic passive sampler can extract ionizable compounds from sediments and suspended particulate matter in a pH‐dependent manner [101] . The authors tested the partitioning of ionizable compounds between PDMS and water (log D PDMS/w ) at different pH values.…”

“…This application demonstrated that the thermodynamic equilibrium derived in Equation 2 applies to partitions other than the n ‐octanol/water system, such as in the PDMS/water phases. The presence of some free silanol groups, combined with the highly hydrophobic polymeric chain might create a suitable environment for ionized‐organic species in the PDMS phase through a combination of hydrogen bonds from the terminal −OH groups and dispersion non‐covalent forces from the polymeric chains [101,134] …”

“…The experimental pK a values, logD PDMS/w , at several pH values including extreme ranges (from which we were able to obtain logP N and log P app IP ) were provided by the article. [101] Therefore, predictions of the distribution coefficients in the PDMS/water system to pH = 7.4 using Eq. 1 and Eq.…”

“…This hydrophobic passive sampler can extract ionizable compounds from sediments and suspended particulate matter in a pH-dependent manner. [101] The authors tested the partitioning of ionizable compounds between PDMS and water (logD PDMS/w ) at different pH values. Thus, logD PDMS/w measurements at extreme pH were considered as logP N or log P app IP depending on the acidic or basic nature of each molecule.…”

Human Apparent Volume of Distribution Predicts Bioaccumulation of Ionizable Organic Chemicals in Zebrafish Embryos

“…This contradiction to common perception for the partitioning of IOCs has also been reported recently. For example, Niu et al 51 examined the pH-dependent partitioning behaviors of IOCs between water and polydimethylsiloxane polymer and found partitioning of the charged species of IOCs was smaller than their neutral and zwitterionic forms, but this difference was not negligible.…”