Acidic Constant of Hydrofluoric Acid in High-Pressure and High-Temperature Geofluids

High Resolution Image Download MS PowerPoint Slide Fluorine is a key component in many hydrothermal ore-forming fluids, commonly occurring as fluorite and in minerals such as apatite, fluorite, and mica. The availability of fluoride ions (F – ) for forming metal fluoride complexes is governed by the equilibrium thermodynamics of hydrofluoric acid [HF(aq); Migdisov et al. Chem. Geol. 2016, 439, 13–42 and Xing et al. Geofluids 2018, 2018 (1), 6835346]. However, obtaining reliable thermodynamic properties of HF(aq) at high pressure–temperature ( P – T ) conditions, typical of the Earth’s crust and upper mantle, has proven challenging due to limitations in experimental techniques. This study employs an automated machine-learning molecular dynamics (MLMD) approach that is trained on ab initio data [Wang et al. J. Chem. Phys. 2022, 157 (2), 24103 and Wang et al. J. Am. Chem. Soc. 2024, 146 (21), 14566–14575] to calculate the p K a of HF(aq) across a wide range of temperatures (150–600 °C) and pressures (500 bar to 20 kbar). The calculated p K a values were then fitted into the Ryzhenko–Bryzgalin thermodynamic model with a density dependence to derive properties that can predict the role of HF(aq) in controlling the element mobility and pH in deep-Earth fluids. These results suggest that HF(aq) is a stronger acid than previously estimated, particularly under supercritical conditions, leading to enhanced dissociation and increased availability of fluoride ions (F – ), which may improve the solubility and transport of metals such as rare-earth elements and uranium.