# Molecular-Scale Insights into the Interactions between Perfluoroalkyl Substances and Polyethylene

## TL;DR

This study explores how perfluoroalkyl substances stick to polyethylene plastics at the molecular level, showing that they can act as long-term storage for these pollutants.

## Contribution

The paper provides novel molecular dynamics simulations revealing how PFAS adsorb and orient on polyethylene, comparing semicrystalline and crystalline structures.

## Key findings

- PFOS interacts more strongly with polyethylene than PFOA due to differences in functional group chemistry and chain length.
- Molecular orientation of PFAS changes at the PE-water interface, influenced by polymer structure and local free-energy barriers.
- Microplastics may serve as reservoirs for PFAS, increasing their persistence and transport in the environment.

## Abstract

Microplastics (MPs) and per- and polyfluoroalkyl substances
(PFAS)
are two classes of highly persistent contaminants that frequently
co-occur in the environment, raising concern about potential synergistic
effects. To better understand their interactions, we investigated
the adsorption of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic
acid (PFOS) on polyethylene (PE) through molecular dynamics (MD) simulations.
The potential of mean force (PMF) at infinite dilution was calculated
for both the semicrystalline and crystalline PE models. For semicrystalline
PE systems, the PMF minima were −26.5 ± 4.8 kJ mol–1 for PFOA and −43.9 ± 4.3 kJ mol–1 for PFOS, whereas, for crystalline PE, the values were −26.6
± 5.2 and −42.0 ± 7.7 kJ mol–1, respectively. These results indicate that, within
statistical uncertainty, no significant differences are observed between
the two PE morphologies for either PFAS when considering the depth
of the free-energy minimum. Moreover, PFOS exhibited stronger interactions
with PE than PFOA. This behavior reflects not only differences in
fluoroalkyl chain length but also the distinct chemical nature of
the functional groups, with the larger and more hydrophobic sulfonate
headgroup of PFOS compared to the carboxylate group of PFOA. In addition
to adsorption strength, molecular orientation at the PE–water
interface was characterized. PFAS tails showed a general tendency
to align parallel to PE chains within the polymer slab, but this alignment
was disrupted upon the transition into water. Notably, PFOS interacting
with semicrystalline PE exhibited orientation changes with transitions
between parallel and perpendicular alignment associated with local
PMF barriers. These orientation-dependent interactions highlight the
importance of both chain packing and functional group chemistry in
driving PFAS–polymer affinity. Taken together, these findings
provide molecular-scale evidence that microplastics can act as reservoirs
for PFAS, potentially enhancing their environmental persistence and
transport.