Electric Double Layer at the Rutile (110) Surface. 4. Effect of Temperature and pH on the Adsorption and Dynamics of Ions

Adsorption of Rb+, Na+, Sr2+, and Cl– on hydroxylated (110) rutile surfaces was studied by molecular dynamics (MD) simulations. Our previous work was extended to the range of surface charge densities from −0.2 to +0.1 C/m2 (from −0.4 to +0.1 C/m2 for Sr2+) and to temperatures of 25, 150, and 250 °C. These conditions can be linked to experimental surface charge and pH values from macroscopic titrations of rutile powders with surfaces dominated by 110 crystal planes. Simulations revealed that Na+ and Sr2+ adsorb closer to the surface, shifting from predominately bidentate to tetradentate inner-sphere binding with increasing temperature, whereas Rb+ binding is predominately tetradentate at all temperatures. These differences are related to hydration energies, which must be partially overcome for inner-sphere binding and which decrease with increasing temperature and are lowest for Rb+. The interaction of Cl– with the rutile surface is generally less than that for cations because of repulsion by surface oxygen atoms. These MD results provide molecular-level context for the trends observed in our corresponding macroscopic surface charge titrations. Titration curves steepen in the order Rb+ < Na+ < Sr2+, reflecting the adsorption interactions related to ion charge, radius, and hydration energy.