  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
so far this cooperative effect has been inferred indirectly. This work applies
Density Functional Theory (DFT)-based molecular dynamics simulations of
explicit liquid water/mineral interfaces to calculate metal ion desorption free
energies. Monodentate adsorption of Na(+), Mg(2+), and Cu(2+) on partially
deprotonated silica surfaces are considered. Na(+) is predicted to be unbound,
while Cu(2+) exhibits larger binding free energies to surface SiO(-) groups
than Mg(2+). The predicted trends agree with competitive adsorption
measurements on fumed silica surfaces. As desorption proceeds, Cu(2+)
dissociates one of the H2O molecules in its first solvation shell, turning into
Cu(2+)O(-)(H2O)(3), while Mg remains Mg(2+)(H2O)(6). The protonation state of
the SiO(-) group at the initial binding site does not vary monotonically with
cation desorption.
