Interfacial Reactivity and Speciation Emerging from Na-Montmorillonite Interactions with Water and Formic Acid at 200 °C: Insights from Reactive Molecular Dynamics Simulations, Infrared Spectroscopy, and X‑ray Scattering Measurements

Reactive
organic fluid–mineral interactions at elevated
temperatures contribute to the evolution of planetary matter. One
of the less studied but important transformations in this regard involves
the reactions of formic acid with naturally occurring clays such as
sodium montmorillonite. To advance a mechanistic understanding of
these interactions, we use ReaxFF reactive molecular dynamics (MD)
simulations in conjunction with infrared (IR) spectroscopy and X-ray
scattering experiments to investigate the speciation behavior of water–formic
acid mixtures on sodium montmorillonite interfaces at 473 K and 1
atm. Using a newly developed reactive force field, we show that the
experimental IR spectra of unreacted and reacted mixtures can be accurately
reproduced by ReaxFF/MD. We further benchmark the simulation predictions
of sodium carbonate and bicarbonate formation in the clay interlayers
using small-angle and wide-angle X-ray xcattering measurements. Subsequently,
leveraging the benchmarked force field, we interrogate the pathway
of speciation reactions with emphasis on carbonate, formate, and hydroxide
groups, elucidating the energetics, transition states, intermediates,
and preferred products. We also delineate the differences in reactivities
and catalytic effects of clay edges, facets, and interlayers owing
to their local chemical environments, which have far reaching consequences
in their speciation behavior. The experimental and simulation approaches
described in this study and the transferable force fields can be applied
translationally to advance the science of clay–fluid interactions
for several applications including subsurface fluid storage and recovery
and clay–pollutant dynamics.