Eqn. 4 shows that χ is a linear combination of the contributions from the individual interfacial constituents. These are, in our case, the interfacial water molecules, then the surface silanol groups, and then the anions and cations adsorbed to the protonated and deprotonated surface silanol groups, respectively. The χ value from the interfacial water and surface silanol groups should be close to the one obtained at the lowest ionic strength (ultrapure water, 2 µM ionic strength, pH 5.8) condition. We therefore subtract this χ value from the data shown in Fig. and obtain, at least to leading order, the χ values of the ions bound to the interfacial SiO -and SiOH2 + sites (Fig. ). The results indicate non-monotonic behavior and a maximum in χ around 0.1 mM ionic strength for most of the salts we surveyed. We also find a change in the sign of χ at an ionic strength around 1 mM for the divalent chloride salts we studied (Fig. ), and to a lesser extent in the sulfates (Fig. ). This observation would be expected in case of a flip in the net orientation of the radiating dipoles that produce the SHG response based on trends documented in previous studies. We therefore find experimental evidence for a significant change in interfacial structure with increasing surface coverage for some of the ions we surveyed, consistent with reports by others for mica:water and silica:water interfaces. Previous studies of divalent cations, specifically magnesium and calcium, by Gibbs and co-workers, have shown that low concentrations of these salts (0.033mM) attenuate the vibrational sum frequency generation (SFG)-resonant water signal in comparison to NaCl at similar concentrations albeit at a higher pH than the conditions studied here. The resonant vibrational SFG experiments attributed these trends to displacement of the hydration layer above the silica surface by ions retaining their centrosymmetric hydration shell. These trends were attributed to close association of Ca 2+ to the interface, a finding that would be consistent with the non-resonant c estimates reported here (as well as the largest reduction in interfacial potential by CaCl2 relative to NaCl).