Interfacial reactivity and speciation emerging from Na-montmorillonite interactions with water and formic acid at 200 °C: Insights from reactive MD, IR spectroscopy, and X-ray scattering

Summary¶

Muraleedharan et al. develop a ReaxFF description for Na-montmorillonite interacting with water and formic acid at 473 K and 1 atm, training against DFT reaction energetics and validating against FTIR spectra of unreacted and reacted mixtures and against small-/wide-angle X-ray scattering signatures associated with carbonate/bicarbonate products in clay interlayers. The scientific target is interfacial speciation under hydrothermal-relevant conditions: which pathways produce carbonate, formate, and hydroxide species, and how reactivity differs across edge, facet, and interlayer regions where acidity and confinement vary. Adri C. T. van Duin is a coauthor, aligning the paper with the group’s clay–fluid reactive modeling and geo chemistry portfolio.

Methods¶

Experiments (clay + fluid, 200 °C). SWy-3 Na-montmorillonite is reacted with water and 1:1 formic acid/water in Parr vessels at 200 °C (473 K), 2 h, 1 atm; solids are recovered for FTIR, and SAXS/WAXS target interlayer spacing and inorganic product signatures after reaction (carbonate/bicarbonate features in the X-ray data as developed in the paper).

Force-field training (ReaxFF for Na–clay + C/H/O). A ReaxFF parameterization is adjusted so that reaction energetics and vibrational fingerprints from ReaxFF-based IR simulations match DFT reference data and the experimental FTIR for selected clay+fluid compositions (iterative fit protocol in the article). QM reference — DFT reaction energies and mode data as listed in the Methods; N/A in this short note to list every functional/basis line—see SI.

MD application (ReaxFF, ADF and LAMMPS). Production speciation and kinetics use ReaxFF in Amsterdam Modeling Suite (ADF). Ensemble: anisotropic NPT with fixed x, y cell edges and a weak Berendsen thermostat/barostat (0.1 ps temperature damping per the article). Time step: 0.25 fs; velocity Verlet integration. Total trajectory length ~0.6 ns; three independent replicates with different initial geometries to average observables. IR postprocessing uses a ReaxFF/LAMMPS path with NPT runs of up to ~20 ps; the total dipole moment is recorded every 0.5 fs for line-shape analysis. State conditions are matched to the laboratory batch targets T = 473 K, P = 1 atm (after gas-phase pre-equilibration to the target density described in the Methods, prior to reactivity). 3D PBC clay+fluid supercells resolve interlayer, edge, and facets in Figure 1. Electric field, shear, shock, metadynamics — N/A.

Static DFT — N/A as a separate results code path beyond DFT used to build and refine the ReaxFF and to benchmark reaction energies as stated in the article.

Findings¶

IR validation. Simulated ReaxFF-based IR line shapes align with experimental FTIR for unreacted and reacted assemblages after the reparameterization, supporting carbonate, formate, and hydroxide assignments under confinement.

Scattering. SAXS/WAXS-guided validation of inorganic interlayer carbonation (e.g. sodium carbonate/bicarbonate-related features) is used as an independent experimental check beyond IR alone.

Interfacial chemistry. The authors stress that reaction pathways differ between clay edge, tactoid face, and interlayer pores because of pH/acidity gradients and nanoconfinement. N/A in this note to map every reaction name; see Results in pdf_path.

Sensitivity and outlook. The T=200 °C, 1 atm batch chemistry and synthetic MMT assumptions frame where the ReaxFF is tied; geologic diversity outside the fit is a knowledge gap—see ## Limitations.

Limitations¶

Idealized clay cells only partially represent stacking faults and cation substitutions in natural samples. FTIR and scattering validation ties parameters to specific batch chemistry; transfer to other clay sources requires prudence.

Use this entry as a geo/environmental bridge to theme-water-silica-geochemistry; keep experiment-integrated in frontmatter for retrieval filters that prioritize validation workflows.

Relevance to group¶

Showcases ReaxFF reparameterization plus experimental validation for clay–fluid chemistry (van Duin co-author).

Citations and evidence anchors¶

ACS Earth Space Chem. 5, 1006–1019 (2021); DOI 10.1021/acsearthspacechem.0c00286 — Methods §2 and Results on IR/SAXS benchmarks.