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 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 forcefield, we
show that the experimental IR spectra of unreacted and reacted mixture 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 and Wide-Angle X-ray Scattering
measurements. Subsequently, leveraging the benchmarked forcefield, 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
forcefields 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
Interfacial Reactivity and Speciation Emerging from Namontmorillonite Interactions with Water and Formic Acid at 200°C: Insights from Reactive Molecular Dynamics Simulations, Infrared Spectroscopy, and X-ray Scattering Measurements