Extended Data Fig. 5. Microplastic-water interface hydrophobicity quantified by tetrahedral order.

a-b, Average values of order parameters a) q₄ and b) q₃ for water as a function of distance to the WCI. The vertical blue line represents WCI. c-f. Correlation between interfacial water tetrahedral order (q₃, q₄) and chemical activity (APF fluorescence at the particle surface) across six water-plastic interfaces. c-d. Averaged c) q₃ and e) q₄ for waters located on the plastic side of the Willard-Chandler interface (distance to WCI < 0 Å). e-f. Averaged e) q₃ and f) q₄ for waters on the aqueous side within 0–2 Å of the WCI. Hydrophobic surfaces lower tetrahedral order relative to bulk, indicating a more defective interfacial H-bond network. The reduction of tetrahedral order is strongest for PE and PP, consistent with their higher hydrophobicity, whereas PMMA and PET are more bulk-like.