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

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 Kow 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 KPDMS/w of neutral chemicals from the literature and extended the dataset
by 93 neutral chemicals. The existing quantitative structure–activity
relationship between the log Kow and KPDMS/w could be extended with the
measured KPDMS/w linearly to a log Kow 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 DPDMS/w(pH)
could be explained by the fraction of neutral species multiplied by
the KPDMS/w of the neutral species of
these IOCs. Seventeen multiprotic chemicals with up to three acidity
constants pKa also showed a pH dependence
of DPDMS/w(pH) with the tendency that
the neutral and zwitterionic forms showed the highest DPDMS/w(pH). DPDMS/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.