Concerted Metal Cation Desorption and Proton Transfer on Deprotonated Silica Surfaces

The adsorption equilibrium constants of monovalent and divalent cations to material surfaces in aqueous media are central to many technological, natural, and geochemical processes. Cation adsorption-desorption is often proposed to occur in concert with proton transfer on hydroxyl-covered mineral surfaces, but to date this cooperative effect has been inferred indirectly. This work applies density functional theory-based molecular dynamics simulations of explicit liquid water/mineral interfaces to calculate metal ion desorption free energies. Monodentate adsorption of Na⁺, Mg²⁺, and Cu²⁺ on partially deprotonated silica surfaces are considered. Na⁺ is predicted to be unbound, while Cu²⁺ exhibits binding free energies to surface SiO^- groups that are larger than those of Mg²⁺. The predicted trends agree with competitive adsorption measurements on fumed silica surfaces. As desorption proceeds, Cu²⁺ dissociates one of the H₂O molecules in its first solvation shell, turning into Cu²⁺(OH^-)(H₂O)₃, while Mg remains Mg²⁺(H₂O)₆. The protonation state of the SiO^- group at the initial binding site does not vary monotonically with cation desorption.