Fig. 3. Theoretical investigations of the molecular structure–activity relationship at plastic–water interfaces.

a, VIE distributions for OH⁻ obtained from 171, 153 and 184 configurations with converged electronic structure calculations for the PE, PVC and PMMA–water systems, respectively, each including explicit solvent molecules within 6 Å of the ion. The PE and PVC systems exhibit a distinct low-VIE tail (<<130 kcal mol⁻¹), which predominantly arises from undercoordinated OH⁻ configurations at the interface. Inset: a representative snapshot sampled from the low-VIE tail region for water–PE interfaces, involving a direct hydrogen bond to a water with a surface dangling O–H bond (grey). b, Probability distribution of OH⁻ hydrogen-bond coordination within approximately 5 Å of the Willard–Chandler interface (WCI). Compared with PMMA, OH⁻ at the PE–water interface shows a much higher fraction of undercoordinated states (<4 hydrogen bonds), whereas PVC exhibits intermediate behaviour between the two systems. c, Comparison of VIE distributions of OH⁻ at the PE–water interface owing to stronger electric fields. The PE* with a dashed line represents the VIE distribution after removing Na⁺ counterions from the PE–water system owing to a surface charge imbalance at the slip plane derived from the zeta potential. d, Hydrogen bonding to water across the hydrophobic to hydrophilic polymers; details of structural simulations are given in Supplementary Information. e, Simulated ratio of surface water dangling OH groups (defined as water without any acceptor (X) within an intermolecular O···X distance less than 3.5 Å and with an H–O···X angle (β) smaller than 50°) versus chemical activity indicated by APF fluorescence intensity measured at microplastic particle surfaces in Fig. 2b. Both OPLS-AA/SPC/E (purple line) and OPLS-AA/ReaxFF-C-GeM (yellow line) are shown. f, Electrical double-layer structure and potential electron acceptors and sinks at hydrophobic plastic–water interfaces. For example, O₂ and H⁺ may accept electrons from interface through the reactions OH⁻ + O₂ → •OH + •O₂⁻ and OH⁻ + H⁺ → •OH + •H, respectively. See Methods and Supplementary Information for details of the polymer and water models, molecular dynamics simulations, configuration sampling, structural analysis and VIE calculations.

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