These results are in some ways surprising. Previous studies that have explored hydration structure and the point of zero charge of silica indicate that cation identity has an outsize effect on the electrostatics over structure at the interface. Likewise, previous x-ray reflectivity studies have revealed that hydration shell structures play a role in trends of interfacial potential. ions at the interface relative to M + ions with a positive lyotropic effect on heats of adsorption for increasing ionic radius. In these experiments, the observed trends in ΔHads were strongly correlated with hydration properties for the monovalent ions, but did not hold as strongly for the divalent cations. Adsorption phenomena and changes in that behavior for various alkali earth cations were instead attributed to bare ionic radius and ionic potential (or charge/radius ratio). Our findings may corroborate this scenario given differences between cation species in our experiments decrease with increasing surface coverage. Studies of the muscovite/water interface have also demonstrated that divalent ions with larger electron density such as Sr 2+ can adsorb in both fully and partially hydrated states. Different adsorption mechanisms for counterion species with larger electron may explain the larger magnitude surface potential for Sr 2+ and Ba 2+ at 100mM, in spite of similar χ values in comparison to Ca 2+ and Mg 2+ at higher surface coverages.