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The hydrophilic-to-hydrophobic transition in glassy silica is driven by the atomic topology of its surface

Summary

Wetting of silica controls friction, dissolution, and chemical durability in aqueous environments, yet linking silanol coverage to atomistic network topology remains contentious. Yu et al. generate artificial glassy silica surfaces with reactive molecular dynamics using the Pitman Si/O/H ReaxFF parameterization in LAMMPS with the user-reaxc package and charge equilibration. They anneal surfaces to different temperatures to vary rigidity and silanol population, then expose the surfaces to water to follow bond-making and bond-breaking events. Topological constraint theory metrics correlate network flexibility with reactivity and hydrophilicity, arguing that higher annealing temperatures yield more stable, less hydrophilic surfaces through lower silanol density rather than through roughness alone. The study explicitly challenges explanations that ascribe wetting shifts solely to geometric roughness without reference to chemical speciation.

Methods

Simulations employ ReaxFF with QEq charges as described for silica–water chemistry in the article. Glassy silica slabs are prepared with controlled thermal history to modulate surface topology before water contact. Reactive trajectories track dissociative versus molecular water adsorption and subsequent silanol interconversion. Topological constraint theory quantities such as constraints per atom are used alongside chemical descriptors from the trajectories. Analysis couples rigidity percolation ideas with local silanol reactivity metrics extracted from trajectories. Reproducibility requires the cited ReaxFF file, annealing schedules, slab dimensions, water loading, temperature, timestep, and LAMMPS input settings from J. Chem. Phys. DOI 10.1063/1.5010934.

Integrated MD protocol (from article scope; confirm numerics in PDF). Engine: LAMMPS with USER-REAXC and the Pitman Si/O/H ReaxFF parameterization plus QEq charges as reported. Systems: artificial glassy silica slabs with controlled annealing temperature histories, then water exposure to follow dissociative/molecular adsorption and silanol interconversion (atom counts, supercell vectors, and water loadings in Methods). Boundaries: three-dimensional PBC for slab models unless the article specifies otherwise. Ensemble / timestep / duration: NVT/NVE staging and integration timestep in fs, equilibration/production lengths in ps/ns; thermostat coupling (e.g., Nosé–Hoover or Berendsen as implemented for water contact) is specified in the JCP Methods—N/A — exact damping constants and trajectory lengths not transcribed on this page; use pdf_path (corpus extraction_quality: partial plus short local extract). Barostat: N/A — NPT not emphasized for the summarized reactive slab protocol if the article uses fixed-cell NVT for water contact—confirm in PDF. Pressure targets: N/A — bulk GPa/bar stress control unless explicitly reported. Electric field: N/A — external electric bias not part of the wetting study. Enhanced sampling: N/A — umbrella / metadynamics / replica exchange not indicated in the indexed abstract.

Findings

Outcomes and mechanisms. The hydrophilic-to-hydrophobic shift tracks silanol surface density, which the authors tie to atomic topology (topological constraint theory) of the glassy surface rather than to roughness alone. Water adsorption/reaction trajectories link network rigidity metrics to interface reactivity.

Comparisons. The discussion contrasts topology-first explanations with roughness-centric narratives in prior literature on silica wetting.

Sensitivity. Annealing temperature modulates silanol population and rigidity, shifting modeled hydrophilicity along the processing sequence.

Limitations and corpus honesty. ReaxFF parametrization limits quantitative barriers; multicomponent industrial glasses are omitted. This note is grounded in the JCP abstract/extract and pdf_path; pagination-level numerics and full Methods tables must be taken from the PDF because local text is partial.

Limitations

ReaxFF parametrization choices affect quantitative barriers; industrial glasses contain modifiers not modeled here.

Relevance to group

Silica–water ReaxFF surface chemistry benchmark adjacent to 2018jessica-m-rimsza-journal-of-g-chemical-effects fracture study.

Citations and evidence anchors

  • DOI: 10.1063/1.5010934.