With the observed trends in χ among all divalent ions, we compare our findings to previous studies that have examined the effects of increasing ion size on the overall hyperpolarizability in different molecular systems. From our experimental measurements, we find relatively small changes in the second order nonlinear susceptibility between the divalent halide salts. This outcome is surprising given the precedent for relatively large changes in the hydration structure at the interface. Simulations have shown, for example, a highly ordered first solvation shell for divalent cations such as Mg 2+ vs a fairly labile hydration structure for Na + . Furthermore, Na + is predicted to form direct contact ion pairs with silanol groups, whereas Mg 2+ is proposed to not bind directly to silanol groups at the surface, but rather form a hydrogen bonded complex through its tightly bonded hydration sphere. Other MD simulations have shown Ca(OH) + can form at the silica surface upon deprotonation of one of the water molecules in the hydration shell of the Ca 2+ cation. Previous theoretical studies of model complexes have indicated that hyperpolarizabilities generally increase with ionic radius, but our experiments show little changes between ions to such extent. These studies have also found a threefold increase in the hyperpolarizability in these complexes between substitution of Ca 2+ with Na + . However, when examining the normalized 𝜒 (+) values, Ca 2+ and Mg 2+ represent the smallest change in comparison to Na + observed in these experiments. These are also the hardest cations in the series we studied.