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.