Adsorption of Sr 2+ and Ba 2+ at the cleaved mica–water interface: Free energy profiles and interfacial structure

Abstract

Monte Carlo simulations show that the adsorption position of the Sr 2+ or Ba 2+ ion on the cleaved muscovite surface is determined by the radius of the ion’s first hydration shell, hydrogen bonding of the first shell water molecules with the basal oxygens of muscovite as well as a requirement of minimization of the number of muscovite’s lattice cations in the ion’s first coordination shell. Accordingly, Sr 2+ or Ba 2+ adsorbs in ditrigonal cavities at a distance of 1.12 Å or 1.35 Å, respectively, from the basal surface on dehydrated mica and above tetrahedral substitutions at a height of 1.93 ± 0.02 Å or 2.15 ± 0.03 Å, respectively, at the highest simulated water coverage of 28 H 2O per ion. The ion’s displacement from a ditrigonal cavity occurs upon adsorption of 2 H 2O per ion for Sr 2+ and 3 H 2O per ion for Ba 2+. At a coverage of 28 H 2O per ion, outer-sphere adsorption of Sr 2+ or Ba 2+ at a height of 3.9 ± 0.2 Å or 4.17 ± 0.07 Å, respectively, is possible albeit unfavorable on the free energy scale by 107 ± 7 kJ/mol or 89 ± 13 kJ/mol, respectively, as compared to inner-sphere adsorption. Activation energies for the transformation between inner-sphere and outer-sphere adsorptions are calculated to be 121 ± 3 kJ/mol for Sr 2+ and 99 ± 10 kJ/mol for Ba 2+. A comparison of these values with those reported recently for Mg 2+ and Ca 2+ results in an adsorption affinity sequence Mg 2+ > Ca 2+ > Sr 2+ > Ba 2+ in agreement with the sequence predicted recently for low dielectric constant solids (which include mica) ( Sverjensky, 2006). A recent resonant anomalous X-ray reflectivity study of Sr 2+ adsorption on muscovite ( Park et al., 2006) has questioned the common assumption ( Stumm, 1992), which is supported by the present simulation results, that inner-sphere adsorption is stronger than outer-sphere adsorption. A modification of the cleaved muscovite surface as a result of Sr 2+ adsorption in muscovite’s ditrigonal cavities and related destabilization of muscovite’s hydroxyl groups is proposed as a possible reason for this controversy.