Silicate Carbonation Kinetics, Thermodynamics, and Reactivity Thresholds in Nanoconfined Water Films

Chemical transformations in nanoscale water films have poorly-understood constraints and are difficult to quantitatively examine, yet remain relevant across many natural and engineered environments. In most such cases, the roles of water as both a reactant and a solvent can diverge dramatically from bulk liquid due to the highly structured nature of nanoconfined H₂O. In particular, the ordering and limited mobility of interfacial H₂O significantly lowers its dielectric constant and diffusivity relative to bulk water, broadly impacting crystallization, electron transport, sorption, proton transport, H₂O dissociation, carbonic acid generation, and hydration energetics. Forsterite (Mg₂SiO₄) carbonation kinetics in ~1 nm interfacial water films from 35-90 °C were monitored with high-pressure (90 atm) in situ X-ray diffraction, conditions with direct relevance for geologic carbon storage and utilization. We report how monolayer (ML)-scale changes in water film thickness influence the rates, pathways, and apparent activation energies of Mg-carbonate precipitation. Specifically, the apparent activation energy of magnesite (MgCO₃) precipitation (50-90 °C) was determined to be anomalously-low, at only ~35 kJ/mol. This experimentally-derived result indicates nanoconfined Mg²⁺ adopts a hydration configuration that mimics that of aqueous Ca²⁺, in energetics, if not necessarily in structure. Further increases in nanoconfinement from ~5 to ~3.5 ML revealed a previously unrecognized reactivity regime dominated by competing effects of ion hydration and solute diffusion. We also compare the experimental precipitation of Mg-carbonates as a function of temperature and water activity with ab initio thermodynamics calculations. Ongoing molecular dynamics simulations of outer sphere Mg²⁺ complex diffusion and adsorption water film thickness are providing insight into the origin of multiple reactivity thresholds in nanoconfined water films. Lastly, these results are discussed in a quantitative kinetic framework of containing 15 additional olivine carbonation studies, revealing the temperature-dependence of carbonation reaction rates and mechanisms.