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The adsorption equilibrium constants of monovalent and divalent cations tomaterial surfaces in aqueous media are central to many technological, natural,and geochemical processes. Cation adsorption/desorption is often proposed tooccur in concert with proton-transfer on hydroxyl-covered mineral surfaces, butso far this cooperative effect has been inferred indirectly. This work appliesDensity Functional Theory (DFT)-based molecular dynamics simulations ofexplicit liquid water/mineral interfaces to calculate metal ion desorption freeenergies. Monodentate adsorption of Na(+), Mg(2+), and Cu(2+) on partiallydeprotonated silica surfaces are considered. Na(+) is predicted to be unbound,while Cu(2+) exhibits larger binding free energies to surface SiO(-) groupsthan Mg(2+). The predicted trends agree with competitive adsorptionmeasurements on fumed silica surfaces. As desorption proceeds, Cu(2+)dissociates one of the H2O molecules in its first solvation shell, turning intoCu(2+)O(-)(H2O)(3), while Mg remains Mg(2+)(H2O)(6). The protonation state ofthe SiO(-) group at the initial binding site does not vary monotonically withcation desorption.

Concerted Metal Cation Desorption and Proton Transfer on Deprotonated Silica Surfaces