Effect of Coadsorption of Electrolyte Ions on the Stability of Inner-Sphere Complexes

Coadsorption of electrolyte ions can have a marked influence on the adsorption and hence interfacial reactivity of multicharged ions but has generally been overlooked in previous density functional theory (DFT) studies. The impact of this effect is demonstrated for the DFT-based interpretation of in situ FTIR spectra of the controversial binding form of weakly adsorbed carbonate on hydrated hematite nanoparticles and ferrihydrite. Using a new methodology, we show that addition of hydronium or sodium ions in the DFT models leads to assigning the weakly adsorbed carbonate to an inner-sphere monodentate mononuclear complex. FTIR data and the bond length-vibrational frequency correlations established by DFT suggest that the adsorption affinity of ferrihydrite toward carbonate is lower than that of 7 nm hematite NPs. As opposed to the case of carbonate, the outer-sphere adsorption form of sulfate is demonstrated to be stable in the presence of hydronium. These findings have important implications for DFT modeling of the solid-water interfaces, which has become a major complementary tool to interpret spectroscopic and macroscopic observations of adsorption phenomena.