Constrained Surface Complexation Modeling: Rutile in RbCl, NaCl, and NaCF₃SO₃ Media to 250 °C

A comprehensive set of molecular-level results, primarily from classical molecular dynamics (CMD) simulations, are used to constrain CD-MUSIC surface complexation model (SCM) parameters describing rutile powder titrations conducted in RbCl, NaCl, and NaTr (Tr = triflate, CF₃SO₃^–) electrolyte media from 25 to 250 °C. Rb⁺ primarily occupies the innermost tetradentate binding site on the rutile (110) surface at all temperatures (25, 150, 250 °C) and negative charge conditions (−0.1 and −0.2 C/m²) probed via CMD simulations, reflecting the small hydration energy of this large, monovalent cation. Consequently, variable SCM parameters (Stern-layer capacitance values and intrinsic Rb⁺ binding constants) were adjusted relatively easily to satisfactorily match the CMD and titration data. The larger hydration energy of Na⁺ results in a more complex inner-sphere distribution, which shifts from bidentate to tetradentate binding with increasing negative charge and temperature, and this distribution was not matched well for both negative charge conditions, which may reflect limitations in the CMD and/or SCM approaches. In particular, the CMD axial density profiles for Rb⁺ and Na⁺ reveal that peak binding distances shift toward the surface with increasing negative charge, suggesting that the CD-MUSIC framework may be improved by incorporating CD or Stern-layer capacitance values that vary with charge.