Chemicals and materials<\/p>\n
PS microplastic powder with pristine surfaces was purchased with analytical standard grade from Sigma-Aldrich. PS microplastics with different morphologies were purchased from Thermo Fisher and Chem-Impex. The surface purity of model PS microplastics was characterized by X-ray photoelectron spectroscopy (PHI Versaprobe II X-ray Photoelectron Spectroscope) before subsequent experiments. PE, PMMA and PVC were obtained in the format of micrometre-sized powder from Sigma-Aldrich. PET and PP powder were purchased from Goodfellow and Magerials Science, respectively.<\/p>\n
Rh800 was obtained from Sigma-Aldrich. DMPO was purchased from TCI America. Triton\u00a0X-100, methyl orange and ThT were obtained from Thermo Fisher. APF (Thermo Fisher Scientific) was obtained as 5\u2009mM stock in N,N-dimethylformamide (DMF) and freshly diluted with phosphate-buffered saline (PBS; pH 7.4) to working concentrations before imaging for robust fluorescence quantification under a microscope. R-PE (BioLegend) and WGA\u2013Alexa Fluor 647 conjugate (Thermo Fisher Scientific) were used as model charged biomolecules to visualize distribution at plastic surface. Alexa 660-labelled single-stranded DNA (5\u2032-Alexa 660-CAC GCG AAG ACT TAA GAT A-3\u2032) was synthesized and purified by Integrated DNA Technologies.<\/p>\n
All aqueous solutions were prepared using Milli-Q ultrapure water (18.2\u2009M\u03a9\u00b7cm). Solvents including methanol (MeOH), ethanol (EtOH), acetonitrile (MeCN) and DMF were of HPLC grade. Standard buffers and salts (NaCl, FeCl2, NaOH, triethylammonium acetate and trifluoroacetic acid) were purchased from Sigma-Aldrich unless otherwise specified.<\/p>\n
Microscopy consumables, including glass coverslips, were pre-cleaned by piranha solution (3:1 H2SO4:H2O2) followed by copious rinsing with water unless otherwise specified. Disposable cuvettes were used for zeta-potential measurements with a Malvern ZS Nanosizer instrument.<\/p>\n
Methyl orange degradation<\/p>\n
Methyl orange solution (20\u2009\u00b5M) was prepared with different solvents (that is, H2O, MeOH, EtOH, MeCN and DMF) and mixed with PS powder (Sigma-Aldrich) at 10\u2009mg\u2009ml\u22121 concentration. The obtained suspension or solution was kept stirring at room temperature at a stir rate of 900\u2009rpm for 10\u2009days. Aliquots were taken every 2\u2009days to monitor the possible degradation of methyl orange. Suspensions were centrifuged at 16,000\u2009rpm for 3\u2009min to obtain the supernatant. Methyl orange absorbance was calculated as OD465nm\u2013OD700nm \u00a0, where OD465nm and OD700nm\u00a0are the optical densities measured at 465 and 700\u2009nm, respectively. Subtraction of OD700nm\u00a0corrected for background signal, including scattering from residual particles. Supernatants were\u00a0measured in three technical replicate wells\u00a0(100\u2009\u00b5l per well) using a 96-well plate reader.\u00a0Methyl orange solution prepared with the same solvents at the same concentration without PS addition were kept under the same conditions and monitored at the same time as controls.<\/p>\n
ESR measurements<\/p>\n
DMPO was utilized to trap the radicals for ESR spectroscopy detection. To analyse the spontaneous formation of radical species at the PS\u2013water interface, 200\u2009mg PS powder was mixed with 20\u2009mM DMPO solution in water. The suspension was incubated at room temperature for 20\u2009min before filtration to collect the filtrate. The resulting filtrate was transferred into a Wilmad Glass quartz flat cell for ESR spectroscopy measurements. The spectra were acquired at room temperature with a Bruker ELEXSYS E500 EPR spectrometer under the following conditions: microwave power 20.23\u2009mW; 12 scans\u00a0averaged; centre field 3,480\u2009G; sweep width 100\u2009G; modulation amplitude 1\u2009G; microwave frequency 9.768037\u2009GHz.<\/p>\n
APF radical sensing assayConfocal fluorescence imaging<\/p>\n
APF (5\u2009mM; Thermo Fisher) was diluted to 5\u2009\u00b5M using PBS. Then 1\u2009ml of 5\u2009\u00b5M APF was mixed with approximately 1\u2009mg of microplastic\u00a0particles (PP, PS, PE, PET, PVC or PMMA) through vertexing. Aliquots of the mixture were sandwiched in a glass chamber made by an imaging spacer separating the piranha-solution-treated cover glass and glass slide. Confocal fluorescence imaging was performed immediately after sample preparation. The green fluorescence channel was excited with an argon laser at 488\u2009nm and emission was collected within a 505\u2013520\u2009nm band. Time-lapse images were acquired every 3\u2009min with the excitation power optimized to minimize photobleaching and other perturbations during in situ tracking of the fluorogenic APF signal.<\/p>\n
For endpoint quantification of the APF assay, mixtures containing 5\u2009\u00b5M APF and approximately 1\u2009mg of microplastic particles (PP, PS, PE, PET, PVC or PMMA) were first dispersed by vortexing and then incubated in the dark at room temperature for 90\u2009min to allow the fluorogenic reaction to progress at the plastic\u2013water interface. After incubation, aliquots of each sample were mounted in imaging chambers composed of a poly-l-lysine coated coverslip (Neuvitro) and a glass slide separated by a spacer. Fluorescence images were acquired with constant laser power, photomultiplier tube (PMT) voltage, detector gain and pinhole size across all measurements to ensure quantitative comparability. Multiple non-overlapping fields of view were imaged for each polymer type to account for potential spatial heterogeneity in fluorescence signal distribution.<\/p>\n
Quantification of APF fluorescence was performed after generating maximum-intensity Z projections from the acquired confocal stacks. Particle regions of interest were segmented using corresponding stimulated Raman scattering (SRS) images as references to accurately define particle boundaries. The fluorescence intensities were then extracted as area-weighted averages from the segmented regions of interest to represent the relative reactivity of each microplastic type.<\/p>\n
SRS imaging<\/p>\n
We used SRS imaging to characterize microplastics together with confocal imaging analysis54<\/a>. SRS imaging was conducted on a confocal laser-scanning microscope (FV1200, Olympus) integrated with customized laser sources. A commercial laser system (PicoEmerald S, Applied Physics & Electronics) provided both the pump and Stokes beams. The Stokes wavelenghth was fixed at 1,031.8\u2009nm and modulated at 20\u2009MHz by an electro-optic modulator, whereas the pump beam was tunable and set to 784.2\u2009nm to excite C\u2013H vibrational modes characteristic of microplastics. Both beams with an approximately 2-ps pulse width and 80-MHz repetition rate, were spatially and temporally synchronized, and tightly focused onto the sample using a \u00d725 water-immersion objective (XLPlan N, 1.05 NA MP, Olympus). The transmitted beams were collected by an oil-immersion condenser\u00a0with a numerical aperture of 1.4, with the Stokes beam removed by a high-optical-density bandpass filter (890\/220 CARS, Chroma Technology). The transmitted pump beam was detected by a silicon photodiode (S3590-09, Hamamatsu) under 64-V bias.\u00a0The photodiode output was terminated through a 50-\u03a9 load, and demodulated using a high-frequency lock-in amplifier (HF2LI, Zurich Instruments) at 20\u2009MHz. The demodulated pump-loss signal was digitized and transmitted to the FV10 analogue input channel to generate SRS images.<\/p>\n HPLC analysis<\/p>\n An Agilent 1200 series system equipped with a Vydac 218TP-C18 column (250\u2009\u00d7\u20094.6\u2009mm, 5\u2009\u00b5m; Grace) was utilized for HPLC analysis. The gradient was increased from 5% B to 95% B in 15\u2009min (solvent A, water\u2009+\u20090.1\u2009M triethylammonium acetate; solvent B, acetonitrile\u2009+\u20090.1% (v\/v) trifluoroacetic acid). The detection unit was set to monitor the reaction by the absorbance at 488\u2009nm. The peaks corresponding to APF and its oxidation product, fluorescein, were assigned based on the retention time measured from the standard solution.<\/p>\n To examine the APF conversion in water upon PS powder addition, 100\u2009mg PS powder was suspended in 1\u2009ml of 20\u2009\u00b5M APF solution in water with and without 0.1% Triton X-100 supplement. The suspension was left stirring in the dark for 16\u2009h at room temperature before filtration to collect the filtrate for HPLC analysis. A positive control was prepared by mixing 20\u2009\u00b5M APF with 20\u2009mM H2O2 and 2\u2009mM FeCl2 in water and incubated for\u00a0the same\u00a0duration before HPLC analysis.<\/p>\n Surface localization imaging assaySurface localization of charged fluorophore<\/p>\n PS particles (1\u2009mg\u2009ml\u22121) were suspended in 1\u2009\u00b5M Rh800 prepared under different solvent conditions (that is, H2O, 0.1% Triton X-100 in H2O, and EtOH). The suspensions were incubated in the dark at room temperature for 10\u2009min before confocal fluorescence imaging. The far-red channel was excited by an 635-nm laser with a collection band of 655\u2013755\u2009nm.<\/p>\n Surface localization of protein<\/p>\n Plastic particles (10\u2009mg\u2009ml\u22121) were mixed with protein in PBS (0.02\u2009mg\u2009ml\u22121) to image the surface of absorption of the protein under confocal fluorescence imaging. Proteins with different net charges in the PBS environment were selected with R-PE (Biolegend) being negatively charged and WGA (Thermo Fisher, W32466) being positively charged at pH\u20097.4. The mixture was vortexed and incubated in the dark for 15\u2009min before an aliquot of the mixture was sandwiched in a glass chamber in the same way with piranha-solution-treated cover glass. WGA conjugated with Alexa Fluor 647 was imaged in the far-red channel, excited by an 635-nm laser with fluorescence collected between 655\u2009nm and 755\u2009nm. R-PE was imaged in the red channel, excited by a HeNe(G) laser (543\u2009nm) with emission collected between 560\u2009nm and 660\u2009nm. Simultaneous imaging of the two channels revealed the different partition pattern of oppositely charged\u00a0proteins on plastic surfaces.<\/p>\n Surface localization of DNA<\/p>\n Alexa 660-labelled DNA (500\u2009nM; 5\u2032-Alexa 660-CAC GCG AAG ACT TAA GAT A-3\u2032, from Integrated DNA Technologies) was added to plastic particles (approximately 10\u2009mg\u2009ml\u22121) to image the surface adsorption of the DNA molecules under confocal fluorescence imaging. The mixture was vortexed and incubated in the dark for 15\u2009min before an aliquot of the mixture was sandwiched in a glass chamber in the same way with a piranha-solution-treated cover glass for confocal imaging with the far-red channel excited by an 635-nm laser, and emission was collected at 655\u2013755\u2009nm.<\/p>\n ThT assay for protein aggregation analysis<\/p>\n ThT is widely used as a \u2018gold standard\u2019 for selectively staining and identifying amyloid fibril formation both in vivo and in vitro55<\/a>. Here, 0.1\u2009mg\u2009ml\u22121 lysozyme solution stained with 20\u2009\u03bcM ThT in PBS was mixed with plastic particles at a concentration of 1\u2009mg\u2009ml\u22121. The mixture was vortexed before an aliquot was sandwiched in a glass chamber in the same way with a piranha-solution-treated cover glass for confocal fluorescence imaging. ThT fluorescence was excited at 488\u2009nm and collected in the 505\u2013520\u2009nm emission window. Time-lapse imaging was performed on selected particles at 5-min intervals.<\/p>\n Fluorescence images for quantification of the ThT assay for different microplastics were acquired after incubating the mixture in the dark for 2\u2009h. z-stack images were acquired for each region of interest to map the thioflavin fluorescence at the particle surface. Quantification of the fluorescence signal are performed after z-stack projection (maximum intensity) by segmenting the particles in reference to the corresponding SRS images (method described in \u2018SRS imaging\u2019) and measured the area-weighted average\u00a0fluorescence for segmented particle regions\u00a0of interest.<\/p>\n For the experiment comparing the effect of radical scavenger on protein aggregation at the PS\u2013water interface, ThT fluorescence was monitored using a BioTek Cytation 9 Cell Imaging Multimode Reader. Samples containing 4\u2009mg\u2009ml\u22121 PS microplastics, 0.1\u2009mg\u2009ml\u22121 lysozyme and 20\u2009\u03bcM ThT in PBS, with or without 100\u2009mM caffeine, were prepared in a\u00a024-well plate and imaged as z-stacks with a \u00d720 objective before and after a\u00a04-h incubation\u00a0at 37\u2009\u00b0C in the dark inside the instrument. A 4% agarose gel pad prepared in PBS to minimize particle movement in solution during imaging. Five to six fields of view were defined for each condition and the same particle retained within each field at the two time points were identified and analysed. Surface-associated ThT fluorescence was quantified as the average fluorescence intensity over the particle area, and the fluorescence change for each particle was calculated as Ft\/F0, where F0 and Ft denote the surface fluorescence of the same particle before and after incubation, respectively.<\/p>\n Haemolytic assay<\/p>\n Mouse\u00a0whole blood was collected from wild-type mice (BALB\/cJ, 20 weeks old, Jackson Lab) with an animal protocol (AC-AABN0554) approved by the Institutional Animal Care and Use Committee (IACUC) at Columbia University. A final concentration of approximate 10\u2009mM EDTA was supplemented to prevent coagulation. Haemolysis\u00a0was assessed using a published protocol56<\/a>. As soon as possible after phlebotomy, the blood samples were centrifuged at 1,700\u00a0g for 5\u2009min. The supernatant was removed by aspiration and 2\u2009ml PBS pH\u20097 was added. The washing step was repeated three times or until the supernatant was clear. After final aspiration, the remaining pellet was diluted in PBS\u00a0(1:100) to obtain erythrocyte suspension. To test haemolytic activity, 150\u2009\u03bcl of obtained erythrocyte suspension was gently mixed with 2\u2009mg plastic particles and incubated for 60\u2009min at 37\u2009\u00b0C without agitation before centrifuged at 1,700g for 5\u2009min to collect the supernatant. The same erythrocyte suspension with no plastic particles was examined as a negative control. Haemolytic activity was then quantified by measuring\u00a0optical density\u00a0at 405\u2009nm\u00a0(OD405nm)\u00a0of the supernatant using a plate reader with 96-well plates.<\/p>\n Zeta-potential measurement<\/p>\n The electrophoretic mobilities of PS particles in different conditions were measured using a Malvern ZS Nanosizer instrument. Pristine PS microparticles or carboxylic-acid-functionalized PS microparticles were prepared with a concentration of 1\u2009mg\u2009ml\u22121 in 1\u2009mM NaCl aqueous solution to examine the sensitivity of their surface electrostatics to different concentrations of Triton X-100 supplement. Pure water and ethanol were also used to suspend the pristine PS microparticles or carboxylic-acid-functionalized PS microparticles at the same concentration to compare their solvent sensitivity to the surface electrostatics. Zeta-potential (\u03b6) values were calculated from the electrophoretic mobility values using the following expression: \\(\\mu =\\frac{\\varepsilon {\\varepsilon }_{0}}{\\eta }\\zeta f(\\kappa a)\\), where \u03b50 is the vacuum permittivity, \u03b5 is the relative permittivity of solvent, \u03b7 is the viscosity of solvent and f(\u03baa) is Henry\u2019s function. The H\u00fcckel formula was used to calculate the zeta potential\u00a0and compare between different samples15<\/a>. All measurements were performed at 25\u2009\u00b0C with disposable plastic cuvettes.<\/p>\n Theory and simulationNon-reactive and reactive force fields<\/p>\n For many of the structural analyses used in this study, we used the force-field combination of OPLS-AA for the polymer and the SPC\/E water model. However, to generate configurations for the VIE calculations, we used a more sophisticated ReaxFF\/C-GeM model to describe water, OH\u2212 and Na+. Previous studies have shown that classical fixed-charge OH\u2212 force fields often predict overcoordinated hydration environments, frequently yielding coordination numbers exceeding six. In contrast, ab initio simulations and experimental evidence indicate that hydroxide is best described as a hypercoordinated species with approximately three to five hydrogen bonds. In this context, reactive force-field approaches such as ReaxFF\/C-GeM provide a more physical description by explicitly accounting for charge redistribution and proton-transfer dynamics. ReaxFF\/C-GeM has been shown to accurately capture the energetics and dynamics of hydronium and hydroxide ion species57<\/a>. In particular, proton-transfer events that interconvert different hydration states of hydroxide (for example, transitions between four- and three-hydrogen-bond configurations) have been identified as rate-controlling steps for hydroxide diffusion. ReaxFF\/C-GeM has been shown to reproduce both the solvated structure of hydroxide and its diffusion coefficient in water with reasonable accuracy (for example, approximately 0.40\u2009\u00c52\u2009ps\u22121 from ReaxFF\/C-GeM compared with approximately 0.56\u2009\u00c52\u2009ps\u22121 experimentally). Such capabilities are especially relevant for resolving interfacial hydration structure and ion mobility at polymer\u2013water interfaces, which are central to the present study.<\/p>\n Electrostatic embedding model for ReaxFF\/C-GeM<\/p>\n In this study, we performed MD simulations of an interfacial system of an aqueous NaOH solution with three microplastic systems in a slab geometry using an electrostatic embedding scheme by modelling the water and NaOH with the reactive force-field ReaxFF\/C-GeM57<\/a>,58<\/a>, and used the non-reactive OPLS-AA59<\/a>,60<\/a> for PE, PVC and PMMA. The system is partitioned into three components:<\/p>\n 1.<\/p>\n ReaxFF\/C-GeM atoms, which include all water and NaOH and are treated using the reactive force field.<\/p>\n 2.<\/p>\n All polymer atoms are described entirely by a non-reactive (OPLS-AA) force field.<\/p>\n 3.<\/p>\n Non-bonded interactions between the ReaxFF\/C-GeM and OPLS-AA regions, including van der Waals and electrostatic interactions, which provide the coupling between the two force-field descriptions. For the van der Waals interactions, we adopt parameters consistent with the OPLS-AA\/SPC\/E force field, following the same protocol established in our previous work on reverse micelle systems61<\/a> and using the standard mixing rules of the OPLS force field. Water\u2013polymer electrostatics are treated with fixed charges for the polymer and Gaussian charge density for the water and NaOH. See refs. 61<\/a>,62<\/a> for further details.<\/p>\n Importantly, this electrostatic embedding modelling strategy has been used and validated in previous studies, including systems involving both neutral and charged reverse micelles61<\/a>,62<\/a>. In those studies, the force-field combination was benchmarked against a range of experimental observables from dielectric relaxation and terahertz spectroscopy related to interfacial proton-transfer-related phenomena61<\/a>,62<\/a>.<\/p>\n Further benchmarking of force-field combinations<\/p>\n We have also evaluated interaction energies and hydrogen-bond-related intermolecular distances for representative polymer\u2013water model systems (Supplementary Table 1<\/a>), constructed from small-molecule analogues and optimized consistently at each level of theory. As shown in Supplementary Table 1<\/a>, both OPLS-AA\/SPC\/E and OPLS-AA\/ReaxFF-C-GeM reproduce the correct qualitative trends in interaction strength across the different systems. Notably, the OPLS-AA\/ReaxFF-C-GeM model shows improved agreement with density functional theory in both binding energies and intermolecular distances, particularly for PMMA, where stronger hydrogen-bonding interactions are more accurately captured. These results support that the hybrid OPLS-AA\/ReaxFF-C-GeM approach provides a physically reasonable and reliable description of polymer\u2013water interactions at the interface.<\/p>\n In addition, radial distribution functions between water and representative polymer surface atoms further confirm that both force-field descriptions yield consistent interfacial hydration structures, with only minor quantitative differences, as seen in Extended Data Fig. 7a\u2013f<\/a>. The two force-field descriptions (OPLS-AA\/SPC\/E and OPLS-AA\/ReaxFF-C-GeM) yield overall consistent radial distribution function profiles across the three systems. For the PMMA system, the OPLS-AA\/ReaxFF-C-GeM model exhibits a more pronounced first peak, indicating stronger water\u2013polymer interactions. This is consistent with interaction energy analysis, which shows that hydrogen-bonding interactions are stronger in the OPLS-AA\/ReaxFF-C-GeM description compared with OPLS-AA\/SPC\/E. Notably, this stronger interaction is in closer agreement with density-functional-theory-level calculations, suggesting that the ReaxFF\/C-GeM-based description provides a more accurate representation of interfacial hydrogen bonding in this system.<\/p>\n Simulation details for OPLS-AA\/SPC\/E used for structural analysis<\/p>\n For water\u2013plastic interfaces, plastics and Triton X-100 surfactant molecules were modelled with the all-atom OPLS-AA force field59<\/a>,60<\/a> and water using the SPC\/E model. Each system consisted of a single long polymer chain in a slab geometry of approximately 40,000 polymer atoms and approximately 9,000 water molecules. Non-bonded interactions were truncated at 10\u2009\u00c5, and for SPC\/E long-range electrostatics were computed using the particle\u2013particle particle-mesh Ewald method with a convergence criterion of 10\u22125. The molecular dynamics simulations were carried out using LAMMPS63<\/a>, and used a 1-fs timestep with SHAKE constraints applied to SPC\/E water molecules64<\/a>. The temperature was maintained at 300\u2009K using a Nos\u00e9\u2013Hoover thermostat (damping constant 100\u2009fs)65<\/a>, and constant pressure in the z direction (1\u2009atm) was enforced.<\/p>\n The initial polymer slabs were prepared following the protocol previously described44<\/a>. The polymer systems were first equilibrated for 500\u2009ps before introducing SPC\/E water. After adding water above the slab, the system was further relaxed and equilibrated for 1\u2009ns, followed by a 5-ns production run. Snapshots were saved every 10\u2009ps, yielding 500 frames for structural analysis presented in Fig. 3d,e<\/a>, and in Extended Data Figs. 4<\/a>\u20136<\/a> and 7a\u2013f<\/a>.<\/p>\n Simulation details for OPLS-AA\/ReaxFF-C-GeM used for structural analysis<\/p>\n We performed\u00a0molecular dynamic simulations\u00a0with the electrostatic embedded model using a modified version of LAMMPS for PE, PVC and PMMA58<\/a>. Each system consisted of a single long polymer chain in a slab geometry of approximately 8,000 polymer atoms and approximately 2,000 water molecules for ReaxFF\/C-GeM. Non-bonded interactions were truncated at 10\u2009\u00c5. The simulations used a 0.25-fs timestep, the temperature was maintained at 300\u2009K using a Nos\u00e9\u2013Hoover thermostat (damping constant 100\u2009fs)65<\/a> and a constant pressure in the z direction (1\u2009atm) was enforced. The initial polymer slabs were prepared following the protocol previously described44<\/a>. The polymer systems were first equilibrated for 500\u2009ps before introducing water. After adding water above the slab, the system was further relaxed and equilibrated for 400\u2009ps using ReaxFF\/C-GeM, followed by a 2-ns production run. Snapshots were saved every 5\u2009ps, yielding 400 frames for ReaxFF\/C-GeM for structural analysis in Fig. 3e<\/a> and in Extended Data Figs. 7<\/a> and 8<\/a>.<\/p>\n Simulation quantities for the microplastics<\/p>\n To better characterize the polymer surfaces in our OPLS-AA simulations, we analysed both the polymer density and the distribution of exposed functional groups at the interface. Supplementary Table 2<\/a> shows that the equilibrated microplastic systems reproduce physically reasonable bulk densities using a standard 21-step protocol66<\/a>. We also computed the surface-exposed functional groups density profile, \u03c1poly(z), to identify the interfacial region. Within this region, surface-exposed functional groups were defined as those located within 4\u2009\u00c5 of the Willard\u2013Chandler interface (WCI, the instantaneous interface defined as the isodensity surface of the water oxygen density field at half of the bulk liquid density). We then evaluated the surface density of functional groups for each polymer system. As PE does not contain functional groups, it is not included in this analysis. For the other systems, we selected representative groups: chlorine atoms for PVC, the two oxygen atoms in the COOR group for PET and PMMA, and the aromatic ring centroid for PS. As shown in Extended Data Fig. 8a<\/a>, the surface functional group densities exhibit narrow distributions, with relative fluctuations below approximately 5%, indicating that the exposed surface composition remains stable over the course of the simulation, without significant interfacial reconstruction. Furthermore, comparisons between OPLS-AA\/SPC\/E and OPLS-AA\/ReaxFF-C-GeM show only minor differences in the distributions, suggesting that the interfacial functional group exposure is robust with respect to the choice of water model. These results indicate that, although the polymer matrices are amorphous, the interfacial surface structure remains statistically stable on the simulation timescale, supporting the reliability of the observed hydration and reactivity trends. Together, these results support the use of a\u00a0single-chain representation for the present interfacial study.<\/p>\n VIE calculations and sampling details<\/p>\n For VIE calculations, 4 NaOH molecules were added to the water phase (box size approximately 3.6\u2009\u00d7\u20093.6\u2009\u00d7\u20097\u2009nm3) based on the surface charge density reported in ref. 42<\/a>. VIEs were computed following the procedure in ref. 19<\/a> at the \u03c9B97M-V level of theory using Q-Chem67<\/a>, with aug-cc-pVTZ for OH\u2212\/OH\u2022 and aug-cc-pVDZ for surrounding environments. Each calculation included explicit solvent within 6\u2009\u00c5 of the hydroxide ion, with the remainder treated via charge embedding from atomic charges from the remaining simulated environment. Hydrogen bonds to OH\u2212 were defined by a donor(water-H)\u2013acceptor(O\u2013H\u2212) distance \u22642.7\u2009\u00c5 and an O\u2013H\u2013O angle \u2265115\u00b0. All results are shown in Fig. 3a<\/a>.<\/p>\n As the experimentally measured zeta potential reflects the potential at the slip plane, this implies a net charge imbalance within the interfacial region, that is, the particle effectively carries excess negative charge. To model this, we removed \u00a0all Na+ ion as a practical way to reproduce this charge imbalance in a finite simulation set-up. Explicitly resolving the electrical double layer would require substantially larger system sizes and longer timescales than are currently feasible. Removing Na+ therefore provides a simple but direct way to impose an experimentally motivated interfacial charge density consistent with experimentally measured zeta potentials. All results are shown in Fig. 3c<\/a>.<\/p>\n In our VIE calculations, configurations are constructed by including explicit solvent molecules within 6\u2009\u00c5 of the hydroxide ion, which may exclude direct polymer\u2013OH\u2212 contacts in some interfacial configurations. To assess the impact of this approximation, we directly compared VIE values computed with and without configurations involving the OH\u2212 proximity to the PE plastic. As shown in Extended Data Fig. 7g<\/a>, the two sets of results are in close agreement on a per-configuration basis, with data points distributed near the parity line. The average VIE values are 192.99\u2009kcal\u2009mol\u22121 and 193.82\u2009kcal\u2009mol\u22121, differing by only 0.83\u2009kcal\u2009mol\u22121 (<0.5%). These results indicate that excluding polymer atoms from the VIE region does not materially affect the computed ionization energies and that the VIE is primarily governed by the local solvation environment of hydroxide.<\/p>\n To ensure consistency and convergence across systems, all molecular dynamics simulations using ReaxFF\/C-GeM are over a timescale of 2\u2009ns for each polymer\u2013water system. The time-resolved VIE distributions show that the statistics are well converged over the simulation timescale, whereas the spatial distributions of undercoordinated hydroxide ions relative to the interface are consistent with the observed trends in VIE behaviour across different polymer systems. The results shown in the main text (Fig. 3a,c<\/a>) were obtained from one representative 500-ps trajectory window, from which configurations were sampled every 2.5\u2009ps. By contrast, Extended Data Fig. 8c\u2013e<\/a> is included primarily to demonstrate convergence across the full 2-ns simulation. For this analysis, the trajectory was divided into 4 consecutive 500-ps windows, and configurations were sampled every 5\u2009ps within each window, yielding approximately 100 configurations per segment. Owing to occasional SCF convergence failures in the electronic-structure calculations, the number of successfully evaluated configurations is slightly reduced but remains sufficient to construct statistically meaningful VIE distributions. To improve the sampling of rare interfacial configurations, we additionally used a targeted sampling scheme based on structural criteria (distance to the WCI <5\u2009\u00c5 and hydrogen-bond coordination number <4), with configurations selected at 1-ps intervals. As shown in Extended Data Fig. 8c\u2013e<\/a>, the time-resolved VIE distributions reveal distinct behaviours across the three polymer systems. In the PE system, a low-VIE tail is observed in both the early and late trajectory segments, indicating that such configurations are sampled repeatedly over the course of the simulation. In the PVC system, a low-VIE tail emerges in the later trajectory segment (1.5\u20132.0\u2009ns), consistent with the behaviour observed in PE, but with lower frequency, reflecting the presence of rare interfacial configurations. In contrast, as shown in Extended Data Fig. 8b<\/a>, the PMMA system exhibits a substantially lower population of undercoordinated hydroxide ions in the interfacial region, and no configurations corresponding to the low-VIE tail are observed over the full 2-ns simulation. These observations further support the connection between interfacial hydroxide structure and VIE behaviour across different polymer systems.<\/p>\n","protected":false},"excerpt":{"rendered":"Chemicals and materials PS microplastic powder with pristine surfaces was purchased with analytical standard grade from Sigma-Aldrich. 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