Ion-exchange mechanisms and interfacial reaction kinetics during aqueous corrosion of sodium silicate glasses

The study addresses how sodium silicate glasses undergo aqueous corrosion at the atomic scale, focusing on fundamental ion-exchange mechanisms (Na+/H+) and associated interfacial reactions that govern hydration, hydrolysis, and the evolution of alteration layers. Understanding these processes is essential for predicting performance and durability of glasses in environments involving water (from consumer products to nuclear waste immobilization). The research aims to resolve mechanistic details of proton transport, silanol formation and re-formation, and the role of water in mediating reactions, which occur at nanometer length scales and nanosecond to long timescales and are challenging to observe experimentally.

The paper reviews experimental characterization of glass-water interactions (NMR, SIMS, XPS, FTIR, Raman) and computational approaches (AIMD, classical MD, MC). Classical MD has elucidated bulk silicate glass structures, but reactive interfacial chemistry requires potentials allowing bond breaking/forming. Earlier potentials (e.g., Du & Cormack for silica surfaces; CLAYFF) did not capture water dissociation. Reactive potentials enabling dissociation include Mahadevan-Garofalini’s dissociative water potential and ReaxFF. Recently refined ReaxFF parameters for Na–Si–O–H incorporating first-principles data have enabled accurate simulation of sodium silicate structures, sodium diffusion energetics, and glass-water reactions. Prior studies indicated NBO protonation (silanol formation) dominates initial reactions and proton hopping mediates transport in Na silicate, but detailed ion-exchange mechanisms and kinetics remained unclear, motivating this work.

Reactive MD simulations using refined Na–Si–O–H ReaxFF were performed in LAMMPS. Glass composition: 22.8 Na2O–77.2 SiO2 with experimental density 2.365 g/cm³. Model generation: (1) Bulk glass of 8001 atoms formed via melt-quench using partial charge pairwise potentials, then relaxed with refined ReaxFF; (2) Two surfaces created by elongating the box in one direction and annealed at 300 K for 100 ps (NPT). Timesteps: 1.0 fs for glass formation/surface relaxation. Thermostat/barostat: Nosé–Hoover with damping 100× and 1000× the timestep, respectively. (3) Water box with 3000 H2O molecules (density 1.0 g/cm³) generated with ReaxFF, relaxed at 300 K (NVT), and placed atop the glass with a 3–5 Å gap on both surfaces using a 0.1 fs timestep to avoid spurious reactions. Interfacial reaction simulations: initial relaxation 10 ps at set temperatures (300, 350, 400, 450 K) under NPT, followed by NVT production runs up to 3.0 ns with 0.1 fs timestep. System size held constant for analysis. Concentrations computed using binning in z (1 Å) over the xy box area. ReaxFF energy terms include Coulomb, van der Waals, bond, valence angle, torsion, over-/under-coordination penalties, and lone-pair energies, enabling bond making/breaking. Analyses included tracking O–H and O–Na distances at reactive sites to identify ion-exchange events, trajectory visualization of Na+ and H+, classification of reaction mechanisms (silanol formation; self- and adjacent-exchange re-formation), concentration-time profiles for H2O, NBO, Si–OH, OH−, reaction rate estimation from concentration changes, and frequency counting of mechanism-specific proton transfers between consecutive frames (10 ps interval).

- Mechanisms of ion exchange: Na+↔H+ exchange occurs via (i) self-exchange between two NBOs on the same [SiO4] tetrahedron and (ii) adjacent-exchange between NBOs on neighboring tetrahedra. A clustered configuration where a proton is shared between two NBOs (Si–O–H–O–Si) can persist, mediating transport. - Roles of water: Water acts as (a) an intermediate facilitating proton transfer (forming and breaking silanol groups) and (b) a terminator that resolves clustered/reversible states by supplying/removing protons, enabling net progression of reactions. - Kinetics and staging: Two clear stages within 3 ns. Stage 1 (<~0.25 ns): rapid decrease of H2O and NBO; dominated by water contact/diffusion and fast silanol formation (H2O + Si–NBO → Si–OH + OH−). Stage 2: slower overall rate as water diffusion is limited; increased frequency of reversible silanol re-formation (self and adjacent) reduces net proton penetration rate. - Temperature effects: All reaction frequencies increase with temperature. Accumulated reactions per nm³ over 3 ns (from Table 1): • Silanol formation: 8.83 (300 K), 12.73 (350 K), 15.24 (400 K), 22.89 (450 K) • Self-exchange re-formation: 0.01, 0.01, 0.02, 0.03 • Adjacent-exchange re-formation: 1.95, 4.28, 7.34, 9.37 Silanol formation dominates; adjacent-exchange is second; self-exchange is rare due to limited Si–(NBO)2 units. - Transport characteristics: Protons penetrate rapidly from water into glass and become stabilized as Si–OH; Na+ diffuses more slowly and tends to reside locally near reaction sites before moving toward the surface, consistent with a vacancy-like mechanism and a reported Na+ diffusion barrier of ~0.6–0.7 eV in sodium silicate glass. - Microscopic observations: Ion-exchange events identified by crossover of O–Na and O–H distances (e.g., at ~2.3 ns in representative site). Sodium leaching from reaction zones accompanies formation of stable silanol groups. Complex multi-step sequences can generate multiple silanol groups in adjacent tetrahedra via coupled formation and adjacent-exchange transfers. - Anticipated third stage: With longer times, reactions are expected to enter a slower regime dominated by silanol re-formation, with rare new silanol formation unless new NBOs appear in water-rich regions via proton transport.

The findings clarify how Na+/H+ ion exchange and proton transfer pathways control early-stage hydration and corrosion of sodium silicate glass. Silanol formation is the primary pathway enabling initial proton incorporation at surface NBOs, while reversible self- and adjacent-exchange silanol re-formation processes mediate deeper proton transport. Because Na+ mobility in the dense glass matrix is slower than proton mobility, Na+ often remains near reaction sites, leading to transient reversibility until Na+ migrates (often with nearby OH−) toward the surface, stabilizing newly formed silanols and allowing further reactions to proceed. Elevated temperatures increase reaction frequencies, promoting deeper penetration and enabling more encounters with Si–(NBO)2 units that support self-exchange. Water’s dual role as both mediator and terminator of clustered reactions explains how local hydrous species concentrations can accelerate or stall progression. The staged kinetics inferred from concentration-time profiles align with a transition from diffusion/contact-limited silanol formation to a regime where reversible proton transfers limit net advancement. Differences from acidic environments are anticipated due to increasing local pH (from Na+ and OH− release), which would consume OH− and potentially accelerate silanol formation. Overall, the atomistic trajectories provide direct evidence for ion-exchange mechanisms integral to aqueous corrosion and surface alteration of silicate glasses.

Reactive MD with refined Na–Si–O–H ReaxFF reveals that Na+/H+ exchange in sodium silicate glasses proceeds via proton transfers between NBOs within a tetrahedron (self-exchange) and between adjacent tetrahedra, with water mediating and terminating reactions. Silanol formation dominates the initial (sub‑0.25 ns) stage; reversible silanol re-formation becomes increasingly important afterward and is expected to dominate at longer times. Sodium migration away from reaction sites stabilizes silanol products and facilitates further reaction into the bulk. Clustered configurations where a proton is shared by two NBOs are key intermediates for propagation. Higher temperatures enhance all reaction frequencies, especially silanol formation and adjacent-exchange. These insights provide mechanistic understanding of early-stage hydration and corrosion of silicate glasses. Future work should extend simulations to longer timescales to capture the third kinetic stage, quantify reaction constants and activation energies, and explore effects of solution chemistry (e.g., pH) and multicomponent glass compositions.

- Simulation time limited to 3 ns; later-stage kinetics (anticipated third stage dominated by re-formation) are not directly captured. - Elevated reaction temperatures (300–450 K) were used to accelerate processes; very high temperatures (>500 K) lead to unphysical byproducts, constraining conditions. - Difficulty extracting reliable reaction rates/activation barriers due to coupling of multiple mechanisms and reversibility. - Local pH increases (from Na+ and OH− release) may shift mechanisms compared to acidic solutions; explicit control of solution chemistry was not included. - Model system is sodium silicate (22.8 Na2O–77.2 SiO2); generalization to multicomponent glasses requires further study. - One author affiliation (for index 5) not specified in the provided text.