Changes in adsorption free energy and speciation during competitive adsorption between monovalent cations at the muscovite (0 0 1)-water interface

Abstract

The relationships between the adsorption free energy and speciation of monovalent cations (Li+, Na+, K+, Rb+, and Cs+) at the muscovite (001)-solution interface were investigated using in situ resonant anomalous X-ray reflectivity. The Gibbs free energies of adsorption (ΔGadso) for the cations and the changes in adsorbed Rb+ speciation during competitive adsorption were measured by monitoring changes in the coverage of reference-cation Rb+ as a function of molar ratio between Rb+ and other cations in binary solutions at fixed ionic strengths (0.003m and 0.03m). Larger adsorption free energies (ΔGadso=−22.2±0.7, −23.5±0.6, and −21.2±0.8kJ/mol, respectively) were observed for K+, Rb+, and Cs+, which form mainly inner-sphere (IS) complexes at the interface, compared to those for more strongly hydrated cations Na+ and Li+ (ΔGadso=−16.7±0.6 and −14.3±0.8kJ/mol, respectively) which form dominantly outer-sphere (OS) complexes. The relative coverage of OS Rb+ with respect to IS Rb+ increased with increasing adsorbed coverages of the competing cation. This effect depends on the competing ion, with a larger increase in OS:IS ratio observed with Li+ or Cs+ compared to that with Na+ or K+. The change in proportions of OS to IS Rb+ is explained by systematic perturbation of the interfacial water structure caused by different hydration free energy and hydration structures of the competing cation. These results illustrate the complexity in the speciation and thermodynamics of cation adsorption in natural aqueous systems, and are discussed in the context of the Hofmeister effect.