Interactions of Ca, Zn, and Cd Ions at Buried Solid/Water Interfaces Studied by Second Harmonic Generation

The adsorption of Ca, Zn, and Cd ions at the solid/water interface was studied using second harmonic generation (SHG). Specifically, we used the SHG χ(3) technique to assess and evaluate metal adsorption at a bare fused quartz/water interface, as well as at a carboxylic acid functionalized fused quartz/water interface. Adsorption isotherms were obtained for the metals at each interface and were fit with the diffuse layer and triple layer models to obtain adsorption free energies, interfacial potentials, absolute adsorbate number densities, and interfacial charge densities. The carboxylic acid functionalized interface displayed lower adsorption free energies but higher adsorbed metal number densities than the bare fused quartz/water interface. We suggest that metal binding at the bare fused quartz/water interface is driven by the initial number of deprotonated surface sites and that it proceeds through an outer-sphere mechanism. In contrast, metal binding at the carboxylic acid functionalized interface is associated with carboxylic acid deprotonation at a substantial free energy cost and proceeds directly through metal−carboxylate interactions.