These results can be viewed in light of previous studies that have shown hysteresis in mineral oxide systems depends strongly on water structure as well as surface charge density. Previous calculations of energies of adsorption indicate Ca 2+ likely forms a tighter contact ion pair with the silica surface disrupting the hydration structure at the surface than ions with a larger hydration shell, such as Ba 2+ . These findings may demonstrate why larger hysteretic effects present for the CaCl2 salts compared to the other divalent cations. Surprisingly, we observe the least amount of hysteresis when using NaCl, at least under our present experimental conditions. A previous study we performed on a faster timescale, where the salt concentration was immediately jumped from ultrapure water to 100mM NaCl and vice versa, however, did demonstrate pathdependent effects. The differences between these two outcomes highlight the importance of characterizing each step in surface-specific experiments involving fused silica, water, and salts. Conclusions and Outlook. Divalent cations at the silica/water interface probed with non-resonant HD-SHG spectroscopy measurements reveal that interfacial structure and total potential depend strongly on ion valency. When purely evaluating cation identity, we observe significant differences between the experimentally determined χ value for NaCl and alkali earth cations, but smaller differences between ions of the same valency in that series. These differences are amplified at intermediate salt concentrations, which we attribute to the influence of hydration structure in the Stern layer, which is the origin of χ . Furthermore, we corroborate the differences we report by examining the effects of anion substitution. Finally, we identify that hysteresis in measuring the reversibility of ion adsorption and desorption at fused silica under the stated conditions of our experiments manifests itself both in Stern layer structure, c , and in total interfacial potential, F(0)tot.