Free Energy Relationships in the Electric Double Layer and Alkali Earth Speciation at the Fused Silica/Water Interface

The adsorption of divalent Sr ions at the fused silica/water interface was studied using the Eisenthal χ(3) technique to quantify Sr2+ adsorption at neutral pH as a function of screening electrolyte concentration. We determine binding constants, adsorption free energies, absolute adsorbate number densities, and interfacial charge densities and examine the relationships between the measured adsorption free energies and the electric double layer interfacial potential present at each electrolyte concentration. Our results provide the first direct experimental investigation into the widely used additive adsorption free energy expression in which the observed free energy is modeled as the sum of an electrostatic free energy and an intrinsic chemical free energy. At screening electrolyte concentrations of 10 mM and lower, the free energy for Sr2+ adsorption to the fused silica/water interface depends directly on the interfacial potential, while the observed adsorption free energy becomes independent of the interfacial potential at higher electrolyte concentrations. This change in the free energy/interfacial potential relationship indicates that the charge of adsorbing strontium species changes from +2 to +1 at screening electrolyte conditions exceeding 10 mM. This finding is consistent with the formation of ion pairs which are thermodynamically favored at the interface but not in the aqueous bulk. Additional experiments using bromide and iodide show anion polarizability effects contribute about 10% to the chemical free energy.